ceph-docker-common: detect ceph version

ceph-docker-common: detect ceph version

By detecting the ceph version running in the container we can easily
apply conditions like:
ceph_release_num.{{ ceph_release }} >= ceph_release_num.luminous

We do that already, in ceph-docker-common/tasks/fetch_configs.yml.

This fixes the error:

TASK [ceph-docker-common : register rbd bootstrap key]
******************************************************

fatal: [magna005]: FAILED! => {"failed": true, "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 'dummy'\n\nThe error appears to have been
in
'/home/ubuntu/ceph-ansible/roles/ceph-docker-common/tasks/fetch_configs.yml':
line 2, 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: register rbd bootstrap key\n  ^ here\n"}

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1486062
Signed-off-by: Sébastien Han <seb@redhat.com>