Description of problem:

Tried upgrading cluster from 2.4 release to 3.0, ceph-ansible failed for missing mgr keyring. mgr and its keyring not there in 2.4 but ceph-ansible is expecting it to be there which will be good if we upgrade cluster from 3.0 to 3.x release but not from 2.4 to 3.0

Following is the ceph-ansible failure when upgrading the cluster with mgr.
=========================================

TASK [ceph-mgr : copy mgr key] ******************************************************
task path: /usr/share/ceph-ansible/roles/ceph-mgr/tasks/pre_requisite.yml:33
The full traceback is:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ansible/plugins/action/copy.py", line 457, in run
    source = self._find_needle('files', source)
  File "/usr/lib/python2.7/site-packages/ansible/plugins/action/__init__.py", line 983, in _find_needle
    return self._loader.path_dwim_relative_stack(path_stack, dirname, needle)
  File "/usr/lib/python2.7/site-packages/ansible/parsing/dataloader.py", line 386, in path_dwim_relative_stack
    raise AnsibleFileNotFound(file_name=source, paths=[to_text(p) for p in search])
AnsibleFileNotFound: Could not find or access '~/ceph-ansible-keys/2e342296-a72e-47ab-a200-8cb49b79a2c6/etc/ceph/test.mgr.magna086.keyring'

fatal: [magna086]: FAILED! => {
    "changed": false, 
    "failed": true, 
    "msg": "Could not find or access '~/ceph-ansible-keys/2e342296-a72e-47ab-a200-8cb49b79a2c6/etc/ceph/test.mgr.magna086.keyring'"
}
================================

When tried to upgrade the cluster without mgr observed below fatal message 
================================
TASK [fail if no mgr host is present in the inventory] **********************************************************************************************************************
task path: /usr/share/ceph-ansible/infrastructure-playbooks/rolling_update.yml:38
fatal: [localhost]: FAILED! => {
    "changed": false, 
    "failed": true, 
    "msg": "Please add a mgr host to your inventory."
}
================================

Note: Observed this bug while verifying bz:1494238


Version-Release number of selected component (if applicable):
ceph-ansible version:
[ubuntu@host006 ceph-ansible]$ sudo rpm -qa | grep ansible
ceph-ansible-3.0.9-1.el7cp.noarch
ansible-2.4.0.0-5.el7.noarch

How reproducible:
3/3

Steps to Reproduce:
1. Configure 2.4 cluster 
2. Upgrade 2.4 cluster to 3.0 using ceph-ansible, follow the upgrade procedure as per doc
3. Added mgr to the /etc/ansible/hosts before starting rolling_upgrade

Actual results:
mgr creation fails with expectation of keying exists in cluster.

Expected results:
before proceeding mgr check for mgr service if service available the proceed the normal path but if mgr is going to get created new then it should create the mgr keyring and proceed.

Additional info:
Observed this bug while verifying bz:1494238