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
error if we don't run any tests

This makes subunit-trace error out if it finds that there were no test
results in the subunit stream. This guards against the case where
something breaks the test toolchain and we stop running tests
entirely. Copied over from equivalent code in Tempest where this has
served us well.

Change-Id: I765395335a68dfc7eb754317f8c45c3dfdd85949