Move loadbalancer vip port creation outside of transaction

Currently _create_port_for_vip calls ml2 create_port() method
which includes rpc notification.
That leads to lock wait timeouts in certain cases.

The patch fixes that while making VIP creation process non-atomic.
But that is fine as long until create_vip() returns vip id, it's
not usable from API.

Change-Id: Ie30973de80118a6b022e8c3bb07ca48122ebcd29
Partial-Bug: #1334226
Merge "Move loadbalancer vip port creation outside of transaction"