engine: add comment field to Host [3]

        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: I06e1bec7c4a862813b19073344f48a0ec3c17458
Signed-off-by: Noam Slomianko <nslomian@redhat.com>