Nicira plugin: Do not expect a minimum number of SNAT rules

Nicira plugin: Do not expect a minimum number of SNAT rules

Bug 1192392

The code was assuming a SNAT rule for each subnet was always present
when clearing a gateway. If the SNAT rule was removed for some other
reason from the NVP backend, this would have led to the failure of
the whole operation; however, a failure won't have been necessary
in this case, as the final effect of the operation is to remove
the rules anyway.

Also, this patch ensures a quantum router is put in ERROR state if
it is not found on the NVP backend.

Change-Id: Iac1a61342b490d0d971b884e1aa8d9567b15cac0