engine: Add Label/Unlabel Nic commands

The commands will be used mainly from the restapi
to label or unlabel nics with a specific label.

Change-Id: I73a76c66f4f4c1be2aa59952c50cc9829633d572
Bug-Url: https://bugzilla.redhat.com/1040586
Signed-off-by: Moti Asayag <masayag@redhat.com>