Block IPv6 for services which don't support it

Previously, we allowed FQDNOrIPAddress() for netconsole and kdump, which don't
support IPv6 configuration. Don't allow IPv6 addresses.

Related to this, give users support to disable it by passing an argument to
validators.

Change-Id: Id9dc98d9bc6b11060fd4deefe4332e24c517cb30
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1008841
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1008795
Signed-off-by: Ryan Barry <rbarry@redhat.com>