Reorder operations in create_vip

Reorder operations in create_vip

Previously VIP's port creation was moved outside the transaction
to avoid deadlocks related to rpc calls.
It led to small chance of VIP being fetched by rpc code with
port being still None.

This patch reorders VIP association with the pool so it's done
only after port has been created.

This patch also leaves small possibility of uncaught exception in
case of pool being concurrently deleted in the process of VIP creation

Change-Id: I584558aecc92db4d19fb72b1d006868b840a4d8c
Closes-Bug: #1356227