engine: NetworkValidator uses new validation syntax

Refactor the NetworkValidator to use the new validation synatx for
fluently specifying the validation when possible.

In one instance the usage was with "when" instead of unless, since the
positive case was easier to read than the negative one.

Change-Id: I218166e474c0fb046791f9785f68ac39f4767793
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>