NSX: Fix KeyError in sync if nsx_router_id not found

Previously, a KeyError would occur in the sync code which would
cause the sync thread to stop running. This would occur if there
was a router entry in the database but no nsx_router_mapping and
the router was not found in nsx. Note: this should never happen unless
one did not run the db migration which introduced and migrated the data
for the NeutronNsxRouterMapping table.

Change-Id: I44f3e7de9323f594501db63d3ad33e80e617bfdc
Closes-bug: #1304647