engine: add comment field to Network [4]

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: I935df2acf56afe5fd09249aead7ff6827e360216
Signed-off-by: Noam Slomianko <nslomian@redhat.com>