Default SNAT gateway

As the OpenContrail Neutron plugin supports the Neutron API extension Layer-3 Networking Extension, the plugin should also be able to implement the default SNAT 1:N mechanism between private network linked to a virtual router and the public network set as the gateway of the virtual network. That blueprint propose a mechanism base on the Linux network namespace manage by the l3 agent of Neutron and OpenContrail routing and service instance possibilities.