When using monitor_address_block to determine the ip address of the
monitor node, we need an ip address available in that cidr to be
present in the ansible facts (ansible_all_ipv[46]_addresses).
Currently we don't check if there's an ip address available during
the ceph-validate role.
As a result, the ceph-config role fails due to an empty list during
ceph.conf template creation but the error isn't explicit.

TASK [ceph-config : generate ceph.conf configuration file] *****
fatal: [0]: FAILED! => {"msg": "No first item, sequence was empty."}

With this patch we will fail before the ceph deployment with an
explicit failure message.

Resolves: rhbz#1673687

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>