Call DVR VMARP notify outside of transaction

The dvr vmarp table update notification was being called inside
of the delete_port transaction in ML2, which can cause a yield
and lead to the glorious mysql/eventlet deadlock.

This patch moves it outside the transaction and adjusts it to
use an existing port dictionary rather than re-looking it up since
the port is now gone from the DB by the time it is called.

Closes-Bug: #1377241
Change-Id: I0b4dac61e49b2a926353f8478e421cd1a70be038
Merge "Call DVR VMARP notify outside of transaction"