core: Block attaching unsupported external networks

External networks can't be used on clusters where per-device custom
properties aren't supported, so blocked attaching the networks to such
clusters.

Change-Id: Ia4391fdf62419c9f54f134eaba7a337f4ca0725c
Bug-Url: https://bugzilla.redhat.com/987817
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>