Changes for Neutron migration

* Neutron needs to keep its interface mapp and specify its own API.get()
* The context passed to an API should have an ovsdb_connection attribute
* The helpers.enable_connection_uri function now takes a timeout and an
  execute function as arguments, any kwargs will be passed to that function
* Neutron will need to subclass OvsdbIdl and call vlog.use_python_logger() itself.