Add network-gateway element

Add a new element that allows a gateway device to be set.  Allows
setting a default route other than the one offered by the DHCP lease
from Neutron's DHCP. Useful for when a default route is needed that may
offer access to other networks that may be needed.

Change-Id: I62b874e37070e4ee2e4557fee0bb06faa4264483
Closes-Bug: 1354570