engine: Extract common code for network used

This validation is the same for hosts, VMs and templates; so extracted
it to a common method that handles the core validation logic, and the
calling methods need just to supply the entities and a replacement for
their verbal representation in the message.

Change-Id: I42fdbca64b805cc5d506b2b63add176062b93124
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>