ceph_key: rework container support

ceph_key: rework container support

Previously, we were doing a 'docker exec' inside a mon container, this
worked but this wasn't ideal since it required a mon to be up to
generate keys. We must be able to generate a key without a running mon,
e.g, when we create the initial key or simply when you want to generate
a key from any node that is not a mon.
Now, just like the ceph_volume module we use a 'docker run' command with
the right binary as an entrypoint to perform the choosen action, this is
more elegant and also only requires an env variable to be set in the
playbook: CEPH_CONTAINER_IMAGE.

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