webadmin: Disable clusters incompatible with external networks

Since it is impossible to attach an external network to a cluster that
doesn't support it, it should be reflected in the GUI. While I was at
it, also filtered out non-supporting clusters when importing networks
from provider.

Change-Id: Ida15ca4b63e894ab28f456fdd7e263405d6c679b
Bug-Url: https://bugzilla.redhat.com/1049243
Signed-off-by: Lior Vernia <lvernia@redhat.com>