Description of problem:
Installed Mitaka in a virtualized environment according to https://www.rdoproject.org/testday/mitaka/milestone2/ and https://www.rdoproject.org/rdo-manager/. After rebooting the controller, when trying to access horizon I see: 

503 Service Unavailable
No server is available to handle this request. 


There are exceptions in /var/log/httpd/horizon_error.log:

mod_wsgi (pid=7703): Exception occurred processing WSGI script '/usr/share/openstack-dashboard/openstack_dashboard/wsgi/django.wsgi'.
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 170, in __call__
    self.load_middleware()
  File "/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 52, in load_middleware
    mw_instance = mw_class()
  File "/usr/lib/python2.7/site-packages/django/middleware/locale.py", line 24, in __init__
    for url_pattern in get_resolver(None).url_patterns:
  File "/usr/lib/python2.7/site-packages/django/core/urlresolvers.py", line 401, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/usr/lib/python2.7/site-packages/django/core/urlresolvers.py", line 395, in urlconf_module
    self._urlconf_module = import_module(self.urlconf_name)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/urls.py", line 35, in <module>
    url(r'^api/', include('openstack_dashboard.api.rest.urls')),
  File "/usr/lib/python2.7/site-packages/django/conf/urls/__init__.py", line 33, in include
    urlconf_module = import_module(urlconf_module)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/api/__init__.py", line 36, in <module>
    from openstack_dashboard.api import cinder
  File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/api/cinder.py", line 30, in <module>
    from cinderclient.v2.contrib import list_extensions as cinder_list_extensions 
  File "/usr/lib/python2.7/site-packages/cinderclient/v2/__init__.py", line 17, in <module>
    from cinderclient.v2.client import Client    # noqa
  File "/usr/lib/python2.7/site-packages/cinderclient/v2/client.py", line 16, in <module>
    from cinderclient import client
  File "/usr/lib/python2.7/site-packages/cinderclient/client.py", line 52, in <module>
    from eventlet import sleep
  File "/usr/lib/python2.7/site-packages/eventlet/__init__.py", line 10, in <module> 
    from eventlet import convenience
  File "/usr/lib/python2.7/site-packages/eventlet/convenience.py", line 3, in <module>
    from eventlet import greenio
  File "/usr/lib/python2.7/site-packages/eventlet/greenio/__init__.py", line 3, in <module>
    from eventlet.greenio.base import *  # noqa
  File "/usr/lib/python2.7/site-packages/eventlet/greenio/base.py", line 441, in <module>
    from OpenSSL import SSL
  File "/usr/lib/python2.7/site-packages/OpenSSL/__init__.py", line 8, in <module>
    from OpenSSL import rand, crypto, SSL
  File "/usr/lib/python2.7/site-packages/OpenSSL/rand.py", line 11, in <module> 
    from OpenSSL._util import (
  File "/usr/lib/python2.7/site-packages/OpenSSL/_util.py", line 7, in <module> 
    binding = Binding()
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/bindings/openssl/binding.py", line 63, in __init__
    self._ensure_ffi_initialized()
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/bindings/openssl/binding.py", line 95, in _ensure_ffi_initialized
    cls._register_osrandom_engine()
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/bindings/openssl/binding.py", line 70, in _register_osrandom_engine
    raise RuntimeError("osrandom engine already registered")
RuntimeError: osrandom engine already registered
mod_wsgi (pid=7703): Exception occurred processing WSGI script '/usr/share/openstack-dashboard/openstack_dashboard/wsgi/django.wsgi'.
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 170, in __call__
    self.load_middleware()
  File "/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 52, in load_middleware
    mw_instance = mw_class()
  File "/usr/lib/python2.7/site-packages/django/middleware/locale.py", line 24, in __init__
    for url_pattern in get_resolver(None).url_patterns:
  File "/usr/lib/python2.7/site-packages/django/core/urlresolvers.py", line 401, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/usr/lib/python2.7/site-packages/django/core/urlresolvers.py", line 395, in urlconf_module
    self._urlconf_module = import_module(self.urlconf_name)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/urls.py", line 35, in <module>
    url(r'^api/', include('openstack_dashboard.api.rest.urls')),
  File "/usr/lib/python2.7/site-packages/django/conf/urls/__init__.py", line 33, in include
    urlconf_module = import_module(urlconf_module)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/api/__init__.py", line 34, in <module>
    from openstack_dashboard.api import base
ImportError: cannot import name base


Version-Release number of selected component (if applicable):
python-django-horizon-9.0.0.0b3-dev67.el7.centos.noarch


How reproducible:
100%


Steps to Reproduce:
1. Install Mitaka according to https://www.rdoproject.org/testday/mitaka/milestone2/ and https://www.rdoproject.org/rdo-manager/
2. Reboot the overcloud (I rebooted the host).
3. Wait for the services to come back up and access horizon


Actual results:
503 Service Unavailable
No server is available to handle this request.