restapi: Support labels on host nics

Adding support for labels on host nic level.
The support actions for labels are GET, POST and DELETE
done via:

 /api/hosts/{host:id}/nics/{nic:id}/labels
 /api/hosts/{host:id}/nics/{nic:id}/labels/{label:id}

See: http://www.ovirt.org/Features/NetworkLabels#REST

Change-Id: I127c77382f0a4790d7c9ba40d77577e941e51919
Bug-Url: https://bugzilla.redhat.com/1040586
Signed-off-by: Moti Asayag <masayag@redhat.com>