engine: Block nic labeling if network assigned to other

In case a network that should be attached to a newly labeled
nic is already attached to other nic on the host, the action
should be blocked.

Note that the interfaces are being fetched from the DB in order
to preserve the network bootprotocol and prior configuration.

Change-Id: I4d3d0b179ea1cdc1ed1e3c9901d8b4813130b9ce
Signed-off-by: Moti Asayag <masayag@redhat.com>