Revert "Switch Nova to use oslo.concurrency"

This patch made unit tests stop running (always runs 0 unit tests) we
should revert and put a check in place to prevent that from happening
so that we don't regress the rest of Nova trying to figure out the
oslo.concurrency issue.

This reverts commit 382294762b08abb1921a77b00608ac941478ce05.

Change-Id: I98b04ac2a0ba67ba8af44867799317fdf95b8aff