engine: Add management network validation

This check will make sure that if we're attaching or editing a
management network's attachment on a cluster, then the attachment will
be properly validated (i.e. the network must be required).

Change-Id: I1e035f009796f989072601cc67a51fd8bbd6947b
Bug-Url: https://bugzilla.redhat.com/875488
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>