The Neutron extension for layer2 gateway (networking-l2gw) provides a API to
manage L2GW components. The proposed change is to implement two new Heat
resources to allow management of the L2GW and L2GW-connection resources.
This change implements the first of the two resources,
OS::Neutron::L2Gateway
Change-Id: Ib850f027833106cb39d3d1f6e644bbb1f79f1aac
Task: #19995
Story: #2002150