webadmin: Add gateway & DNS servers columns

Added the columns to the external subnet sub-tab under a network, in
order to reflect to the user what are the current settings of these
fields.

Change-Id: I794ee197f31ef89f51f27074d5a8f2a1ca5e51d6
Bug-Url: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>