mgr: fix mgr keyring error on rolling_update

when upgrading from RHCS 2.5 to 3.2, it fails because the task `create
ceph mgr keyring(s) when mon is containerized` has a when condition
`inventory_hostname == groups[mon_group_name]|last`.
First, this is incorrect because `inventory_hostname` is referring to a
mgr node, it means this condition would have never been satisfied.
Then, this condition + `serial: 1` makes the mgr keyring creating skipped on
the first node. Further, the `ceph-mgr` role tries to copy the mgr
keyring (it's not aware we are running `serial: 1`) this leads to a
failure like the following:

```
TASK [ceph-mgr : copy ceph keyring(s) if needed] ***************************************************************************************************************************************************************************************************************************************************************************
task path: /usr/share/ceph-ansible/roles/ceph-mgr/tasks/common.yml:10
Tuesday 27 November 2018  12:03:34 +0000 (0:00:00.296)       0:11:01.290 ******
An exception occurred during task execution. To see the full traceback, use -vvv. The error was: AnsibleFileNotFound: Could not find or access '~/ceph-ansible-keys/48d78ac1-e0d6-4e35-ab3e-772aea7828fc//etc/ceph/local.mgr.magna021.keyring'
failed: [magna021] (item={u'dest': u'/var/lib/ceph/mgr/local-magna021/keyring', u'name': u'/etc/ceph/local.mgr.magna021.keyring', u'copy_key': True}) => {"changed": false, "item": {"copy_key": true, "dest": "/var/lib/ceph/mgr/local-magna021/keyring", "name": "/etc/ceph/local.mgr.magna021.keyring"}, "msg": "Could not find or access '~/ceph-ansible-keys/48d78ac1-e0d6-4e35-ab3e-772aea7828fc//etc/ceph/local.mgr.magna021.keyring'"}
```

The ceph_key module is idempotent, so there is no need to have such a
condition.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1649957

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

infrastructure-playbooks/rolling_update.yml

@@ -289,7 +289,6 @@
         - containerized_deployment
         - cephx
         - groups.get(mgr_group_name, []) | length > 0
-        - inventory_hostname == groups[mon_group_name]|last
       delegate_to: "{{ groups[mon_group_name][0] }}"
       with_items: "{{ groups.get(mgr_group_name, []) }}"