Restore osapi_volume_workers config option

osapi_volume_worker was introduced when enabling multi-process API
support for Cinder. However, this config option was accidentally
dropped in commit 464277220ddb800ac69562649fde62e5ca9358c4
(https://review.openstack.org/#/c/67657/20/cinder/service.py).
As a result, the value in cinder.conf wouldn't be honored.  This
change restore this config option.

Closes-bug: #1335295

Change-Id: Ib2d8532d3e6d0f48da0bc4dd6f6e64bfff8973ea
Merge "Restore osapi_volume_workers config option"