engine: add comment field to Storage [5]

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: I0fd8a1b83ed330c945025f13abf565fd899563e7
Signed-off-by: Noam Slomianko <nslomian@redhat.com>