create an UNDO_REQUIREMENTS variable

in the gate, we actually need to *not* roll back the requirements,
otherwise tempest can wedge the world because it runs under tox.

Make a variable that we can set to false in the gate to ensure that
we don't roll back the requirements changes.

Change-Id: I2b842ecc3f6e8b917dd721729640000bd7e7fb78
Merge "create an UNDO_REQUIREMENTS variable"