ceph-fetch-keys: refact

This commits simplies the usage of the ceph-fetch-keys role. The role
now has a nicer way to find various ceph keys and fetch them on the
ansible server.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1624962


Signed-off-by: Sébastien Han <seb@redhat.com>