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)?