engine: Allowed creation of bonds in SetupNetworksHelper

The validation used to check that any NIC to which VLAN-tagged
networks were attached existed on the host; this no longer must be
true since we've enabled adding new bonds that hadn't before existed
on the host.

Change-Id: I167758e86b32bd90f09203321f89bf1a5adc7bf6
Bug-Url: https://bugzilla.redhat.com/968190
Signed-off-by: Lior Vernia <lvernia@redhat.com>