US4050: Backport Icehouse Patch 37131 to Havana (RPC Workers)

The following upstream reviews have been back-ported to our cis-havana-staging branch.

https://review.openstack.org/#/c/64419/
https://review.openstack.org/#/c/72565/

In reviewing the commit history, it is possible that we would also want to back–port the following upstream review as it is related to the addition of RPC workers.

https://review.openstack.org/#/c/90011 

The number of RPC workers is configurable in neutron.conf by setting rpc_workers to a value greater than zero.

Implements: rally-user-story US4050