Call _allocate_vr_id outside of transaction

Call _allocate_vr_id outside of transaction

_allocate_vr_id is called from _set_vr_id, which was starting a
transaction before calling it. This caused an error when the retry
logic was triggered inside of _allocate_vr_id since it would
encounter a DB exception and put the transaction into a bad state
that couldn't be used on the retry.

This patch just stops _set_vr_id from starting a transaction because
it didn't serve a purpose. It also stops _allocate_vr_id from allowing
subtransactions when it starts a transaction, since it's retry logic
isn't compatible with them.

Co-Authored-By: Ann Kamyshnikova <akamyshnikova@mirantis.com>
Closes-Bug: #1528201
Change-Id: If7bcae39098f40f5ee9db78d3190bf9fdaf6438b