Disable the option to use NFS or SSH kdump before networking

As title. I wish I had the option to disable the options instead,
but urwid.RadioButton doesn't present that choice. Instead, don't
display the relevant options at all unless networking is configured.

Change-Id: I85cf0eb4084161d8117c1426010774627c7bc163
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=966343
Signed-off-by: Ryan Barry <rbarry@redhat.com>