engine: Prevent removing a labeled network

Removing a labeled network from an interface should
be block if the nic is labeled with the same label.
In that case, in order to remove the network, the
nic should be unlabeled.

Change-Id: I300c0327fcf838b7581ad5a7ec12ee6bd9b9f0b2
Signed-off-by: Moti Asayag <masayag@redhat.com>