Handle Ceph-related steps for using the RBD store

With this change, the api recipe will take care of the ceph-related
things when the default_store attribute is set to rbd:

- install dependency packages as specified in
  openstack[image][platform][ceph_packages]
- create a cephx client keyring file with openstack-common's template
  and the key found in the data bag specified by the newly introduced
  attribute openstack[image][api][rbd][key_name]

Change-Id: I92881e5a27fb5ec56d4f1fb72933749a3e8d8688