Add the missing ceph_mon host groups for gnocchi
Add the missing ceph_mon host groups for gnocchi

when gnocchi use ceph as backend, need set
gnocchi_backend_storage: "ceph", gnocchi will pull keyring
from ceph mon node [0], but the site.yml missing it.

[0]: https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/gnocchi/tasks/ceph.yml#L39

Change-Id: Ib33c26ee1fdfe683e5c9b3ef987d6da2c588116a
Closes-Bug: #1744010

dardelean/kolla-ansible,dardelean/kolla-ansible,dardelean/kolla-ansible