engine: Added fields to subnet

Added gateway and DNS servers list to the ExternalSubnet entity in
order to allow the users to set these fields.

Change-Id: I1b3718b83bcfb588eb97a55ba559d5407b857955
Bug-Url: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>