Description of problem:

Undercloud installation fails for host with disabled IPv6 (https://access.redhat.com/solutions/8709#rhel7disable).

$ openstack undercloud install
(...)
Error: Could not prefetch sysctl_runtime provider 'sysctl_runtime': sysctl parameter net.ipv6.ip_nonlocal_bind wasn't found on this system
Error: Execution of '/sbin/sysctl net.ipv6.ip_nonlocal_bind=1' returned 255: sysctl: cannot stat /proc/sys/net/ipv6/ip_nonlocal_bind: No such file or directory
Error: /Stage[main]/Main/Sysctl::Value[net.ipv6.ip_nonlocal_bind]/Sysctl_runtime[net.ipv6.ip_nonlocal_bind]/val: change from absent to 1 failed: Execution of '/sbin/sysctl net.ipv6.ip_nonlocal_bind=1' returned 255: sysctl: cannot stat /proc/sys/net/ipv6/ip_nonlocal_bind: No such file or directory
(...)
[2016-12-06 08:36:24,076] (os-refresh-config) [ERROR] during configure phase. [Command '['dib-run-parts', '/usr/libexec/os-refresh-config/configure.d']' returned non-zero exit status 6]

[2016-12-06 08:36:24,076] (os-refresh-config) [ERROR] Aborting...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 845, in install
    _run_orc(instack_env)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 735, in _run_orc
    _run_live_command(args, instack_env, 'os-refresh-config')
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 406, in _run_live_command
    raise RuntimeError('%s failed. See log for details.' % name)
RuntimeError: os-refresh-config failed. See log for details.
Command 'instack-install-undercloud' returned non-zero exit status 1


Version-Release number of selected component (if applicable):
python-tripleoclient-2.0.0-3.el7ost.noarch

How reproducible:
Deploy RHEL, disable IPv6 support as described here: https://access.redhat.com/solutions/8709#rhel7disable
Follow official RHOSP installation guide (https://access.redhat.com/documentation/en/red-hat-openstack-platform/9/paged/director-installation-and-usage/chapter-4-installing-the-undercloud).
Observe installation failing due to the missing kernel parameters for IPv6.

Steps to Reproduce:
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Actual results:
Undercloud installation fails.

Expected results:
Undercloud installation either completes successfully and properly handle missing kernel runtime variables or returns error at the beginning if IPv6 is really needed.

Additional info: