LBaaS: make device driver decide whether to deploy instance

LBaaS: make device driver decide whether to deploy instance

Currently server throws an error in case agent (device driver)
requests logical config for non-active pool, which is a bug
as it's ok if pool is in pending create/update states.
Also the pool may be already scheduled for delete (pending_delete)
while agent requests it to perform some previous update, which
as also ok and agent just doesn't deploy such config.

This patch moves active pool check from server side to agent side

Closes-Bug: #1295491
Change-Id: Ib088355a0b3efffdcd211a8cfe6942833bb9f895