Adds the 'public network' concept to Quantum

Adds the 'public network' concept to Quantum

Implements blueprint quantum-v2-public-networks

This patch allows Quantum to handle public networks. It modifies the
API adding a new attribute to the network resource ('shared')
and enhances the policy engine in order to handle the behaviour of
the service wrt shared networks.
Policy.json specifies a default behaviour which can be changed by
the administrator, even at runtime.
Tests added to test_db_plugin validate 'obvious' behaviour - such as
that only the ports belonging to a given tenant should be returned
even when they are queried on a public network.
Tests added to test_policy instead validate the changes added to the
policy engine.

Change-Id: I0087d449a677ed29357cd3cb4d8580bece940fdf