Implements resource type NetworkGateway

The NVP specific feature "Network Gateway" was added to Neutron in grizzly release.

  https://blueprints.launchpad.net/neutron/+spec/nvp-nwgw-api

This codes aims to add support OS::Neutron::NetworkGateway in heat.

Change-Id: I19aa51c40a216a31d4e5516c08e854a5d02c39a6
Implements: blueprint resource-type-nvp-network-gateway
Merge "Implements resource type NetworkGateway"