Fix parallel unit tests keystoneclient partial checkout

The unit tests were failing with an error that keystoneclient
didn't have some modules (usually v2_0). This was because the
tests were running in separate workers when they should have been
running in a single worker to ensure that the checkout was done by
a single worker.

Change-Id: I33262dd82aaebe8e0ff0f9faf5c5612efb20f551
Closes-Bug: #1301664
Merge "Fix parallel unit tests keystoneclient partial checkout"