Adds support for router rules to Big Switch plugin

Adds support for router rules to Big Switch plugin

Implements: blueprint bsn-router-rules

Adds bigswitch plugin extension which adds 'rules' dictionary to router objects.
Adds validation code and database components to store router rules
Adds configuration option to plugin to set default router rules and max router rules
Adds unit tests to test all router rule functionality
Adds database migration for router rules tables

The Big Switch controller's Virtual Router implementation supports "routing rules"
which are of the form:
<source, destination, next-hop, action>
This extension aims to expose this abstraction via the Big Switch Quantum plugin.

These rules are applied at the router level, allowing tenants to control
communication between networks at a high level without requiring security policies.
(e.g. prevent servers in a publicly accessible subnet from communicating with
database servers).

Change-Id: I37a2740dca93b0a8b5111764458d39f1c2b885ce