engine: External network with vlan must be labeled

An external network which its vlan-id is set, must include a label
when it is being added to the external provider.

Since the engine might not be aware of changes made to the provider,
the label values cannot be enforced.

Change-Id: I811a0d55798b7dd78d37880055c4ac00303b679f
Bug-Url: https://bugzilla.redhat.com/1059249
Signed-off-by: Moti Asayag <masayag@redhat.com>