Create dvr base class and stop passing around snat_ports

Create dvr base class and stop passing around snat_ports

The one thing that I see that the two dvr classes have in common is
the ability to map internal ports to snat ports.  The dvr local router
needs it to set up a redirect to the central part.  The central part
needs it to create the port for the internal network.

This change renames the mapping method to something more logical and
removes snat_ports as an argument to two methods because it is a quick
O(1) operation to get it from the router dict and passing it around
just tangles things up.

Change-Id: Icc099c1a97e3e68eeaf4690bc83167ba30d8099a