Core: Added url_for helpers for Rabbit subcollections (DTACLOUD-526)

This patch add URL helpers for all Rabbit subcollections, like:

system_volumes(:id => system_id)
=>  "http://localhost:3001/cimi/systems/system_id/volumes"

system_volumes(:id => system_id, :ent_id => volume_id)
=>  "http://localhost:3001/cimi/systems/system_id/volumes/volume_id"

Signed-off-by: Michal fojtik <mfojtik@redhat.com>
TrackedAt: http://tracker.deltacloud.org/patch/a5307a0c00bc341d90eb40d575ff73af30dd343f