Fix BadRequest error on add_router_interface for DVR

Fix BadRequest error on add_router_interface for DVR

This operation for DVR is made of multiple steps, some of
which are not within the same DB transaction. For this
reason, if a failure occurs, the rollback will be partial.

This inconsistent state leads the retry logic to fail with
BadRequest, because the router is believed to be already
connected to the subnet.

To fix this condition, it is necessary to delete the port
should the DB deadlock occur.

Closes-bug: #1494114

Change-Id: Ia2a73d6f9d1e4746e761ad072d954e64267a3ad1