NVP plugin: Do backend router delete out from db transaction

NVP plugin: Do backend router delete out from db transaction

Performing the NVP API operation from within a DB transaction
increases the risk of a deadlock between sqlalchemy and eventlet.

With this patch, the operation is moved outside of the db transaction
and appropriate mechanism are put in place for:
i) ensuring neutron db consistency in case of NVP failures
ii) avoiding deleting from backend if neutron logic does not allow it

This patch also synchronizes the routine for removing a router
gateway port from NVP.

Change-Id: I58d156e303e7a56ceb8c62766c192e154b0a3bb4
Closes-Bug: #1258150