roles: ceph-defaults: Check if 'rgw' attribute exists for rgw_hostname

roles: ceph-defaults: Check if 'rgw' attribute exists for rgw_hostname

If there are no services on the cluster, then the 'rgw' could be missing
and the task is failing with the following problem:

msg": "The task includes an option with an undefined variable.
The error was: 'dict object' has no attribute 'rgw'

We fix this by checking the existence of the 'rgw' attribute. If it's
missing, we skip the task since the role already contains code to set
a good default rgw_hostname.

Signed-off-by: Markos Chandras <mchandras@suse.de>