Wrap ML2 delete_port with db retry decorator

Wrap ML2 delete_port with db retry decorator

ML2 delete_port operation currently involves locking ports
and bindings tables which may lead to DBDeadlock errors in certain
cases when several ports are deleted concurrently.
That may happen due to specifics of Galera working in active-active
mode: it may throw deadlock errors when it fails to validate
a change with other members of the cluster.
The fix adds retries to delete port operation to overcome such
deadlocks

Closes-Bug: #1422504
Change-Id: I684691d59c5ac370d74314c3c91857dc709b2d9b