Previously, all subnet properties were updated if any value did not match.
But this behaviour caused errors like e.g. "Current gateway ip 192.168.0.1
already in use by port [ID]. Unable to update." even if that gateway was
not about to be changed.
Task: 40927
Story: 2008172
Change-Id: I049b0dade4c7ea3e1ef24777ae558f650caa136c