common: do not override ceph_release when using custom repo

common: do not override ceph_release when using custom repo

Otherwise it fails like following:

```
TASK [ceph-mds : allow multimds] **************************************************************************************************************************************************
Monday 22 July 2019  16:37:38 +0800 (0:00:03.269)       0:13:25.651 ***********
fatal: [rhel7u6clone1]: FAILED! => {"msg": "The conditional check 'ceph_release_num[ceph_release] == ceph_release_num.luminous' failed. The error was: error while evaluating conditional (ceph_release_num[ceph_release] == ceph_release_num.luminous): 'dict object' has no attribute u'dummy'\n\nThe error appears to have been in '/usr/share/ceph-ansible/roles/ceph-mds/tasks/create_mds_filesystems.yml': line 43, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: allow multimds\n  ^ here\n"}
```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1645379
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>