Distributed gateway for optimized local SNAT

The main functionality difference between the Neutron reference
implementation of SNAT and "Local SNAT", is that with Neutron SNAT the
User reserves an external IP address (from a limited pre-allocated
pool), which is used to masquerades multiple VMs of that same user
(therefore, sharing the same external IP).

With the "Local SNAT" solution, in contrast, the User does not reserve
any external IP in Neutron, and the "external IP" from which each VM
will go out is arbitrarily selected by the underlying networking
infrastructure (similar to the way external IPs are allocated to home
internet routers, or to mobile phones).

The main advantage of "Local SNAT" is the distribution of the NAT/PAT
function to the Compute Nodes, bypassing the Network Node.