Stop nsx sync thread to sync routers at neutron-server startup

DHC-2385

Previous patch to make nsx sync thread stop syncing routers
is not working well. Thread still tries to sync routers
first time the nsx driver is initialized. This patch fix
the incorrect behaviour.

Change-Id: Ided15fd89fffd3f6f0346991daa42b5e727270bd
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>