engine: add comment field to Cluster [2]

    As part of the Free Text RFE

    Added to the entities flow in the system
    DB <-> DAO <-> Entity <-> Model <-> UI
    DB <-> DAO <-> Entity <-> REST

Change-Id: Id3c150cdd66662147070791c85a3c9df59548253
Signed-off-by: Noam Slomianko <nslomian@redhat.com>