tests: change ceph_docker_image_tag for 2nd run

The ceph-ansible upstream CI runs severals tests, including a
'idempotency/handlers' test. It means the playbook is run a first time
and then a second time with an other container image version to ensure the
handlers run properly and the containers are well restarted.
This can cause issues.
For instance, in that specific case which drove me to submit this commit,
I've hit the case where `latest` image ships ceph 12.2.3 while the `stable-3.0`
(which is the image used for the second run) ships ceph 12.2.2.

The goal of this test is not to verify we can upgrade from a specific
version to another but to ensure handlers are working even if it's a valid
failure here.
It should be caught by a test dedicated to that usecase.

We just need to have a container image which has a different id for
the upstream CI, we need the same content in container imagebut a different
image id in the registry since the test relies on image id to decide whether
the container should be restarted.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

tests/functional/centos/7/cluster/ceph-override.json

@@ -5,6 +5,6 @@
 			"osd_pool_default_size": 1
 		}
 	},
-  "ceph_docker_image_tag": "tag-stable-3.0-luminous-ubuntu-16.04",
+  "ceph_docker_image_tag": "latest-bis",
   "ceph_mon_docker_memory_limit": "2g"
 }