Add good path test cases of osapi_compute_workers

While fixing issues in Cinder for bug 1367454 I discovered
that Nova doesn't have any good path test cases to make
sure that settings to osapi_compute_workers is being handled
as expected.

This commit adds those test cases.

Change-Id: I161adf9e93bd7ff1389fdbc8f712ae67ed98b613
Merge "Add good path test cases of osapi_compute_workers"