Description of problem:
ceph-ansible fails at the end of deployment using RGWs if interface is not properly configured with the IP address if radosgw_interface: is set in all.yml only.
The error message is hard to assign to the real problem.


Version-Release number of selected component (if applicable): ceph-ansible-3.1.5-1.el7cp.noarch


How reproducible:
Always.


Steps to Reproduce:
1. configure cluster hosts with IP addresses assigned, except RGWs external interface
2. define radosgw_interface: <interface> in all.yml, not radosgw_address, neither in inventory
3. run ansible-playbook site.yml

Actual results:
TASK [ceph-config : generate ceph configuration file: cephred.conf] ************
Thursday 25 October 2018  14:43:32 -0400 (0:00:00.252)       0:01:42.673 ****** 
fatal: [ceph31-rgw1]: FAILED! => {"failed": true, "msg": "'dict object' has no attribute u'ipv4'"}
fatal: [ceph31-rgw2]: FAILED! => {"failed": true, "msg": "'dict object' has no attribute u'ipv4'"}

PLAY RECAP *********************************************************************
ceph31-rgw1                : ok=40   changed=2    unreachable=0    failed=1   
ceph31-rgw2                : ok=36   changed=2    unreachable=0    failed=1   


Expected results:
ceph-ansible should call out that this would fail missing proper basic setup of hosts

Additional info:
A general pre-check could ease overall process of install and avoid lengthy runs and troubleshooting with poor failure descriptions only.