Add ceph-common to cinder_ceph_packages

The ceph-common packages provides the `ceph` command line client that is
needed by cinder for getting the list of currently active MON hosts:

https://github.com/openstack/cinder/blob/stable/havana/cinder/volume/drivers/rbd.py#L310

Implements: blueprint rbd-for-block-storage
Change-Id: I00c08c26109763923c284e1759527dae911482df