Revert "Add oslo-config-1.2.0a2 and pbr>=0.5.16 to requirements."

This partially reverts commit:
   43b7f22b30244114f809e4c3b44dc337cade95f1.

The oslo.config change in that patch seems to break running unit
tests with tox. This reverts just that part of the change.

Change-Id: I977700d73342e81ee962019b76238d2cb2b1fff4