Fix nsx routers synchronization

Force the our custom nsx driver to not sync nsx routers with
neutron routers

Change-Id: Ia92386db9d33aa37f39e5b7e70f39df3e3c61b92
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>
Don't we also want to manually start our own sync thread here (that makes the router sync a no-op), e.g.,

self._synchronizer = OurSynchronizer(
            self, self.cluster,
            self.nsx_sync_opts.state_sync_interval,
            self.nsx_sync_opts.min_sync_req_delay,
            self.nsx_sync_opts.min_chunk_size,
            self.nsx_sync_opts.max_random_sync_delay)