engine: Consider external networks for VM scheduling

External networks are assumed to be present in the entire cluster that
they're attached to, so if an external network is attached to a vNIC it
shouldn't be required by the scheduler.

Change-Id: I0ea9bf9fadfa9954ee856a38779ed5a627c6165f
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>