webadmin: Add gateway & DNS servers fields

Added the fields to external subnet creation flows to allow users to
specify these values when creating a subnet.

Change-Id: I8c958328e769badebe2d95359d3ae81e657205c2
Bug-Url: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>