Clean up automated changes to requirements

Some of us like to reuse sandboxes, and keep them up to date.  This
is very difficult to do if devstack leaves modifications to
requirements.txt files after a run, since 'git pull' may refuse to
overwrite those changes.

This modification has devstack undo the changes to the requirements
files, to leave the sandbox in a clean state again.

Change-Id: Ia2d928ade8141b59b56a2c4548d760bf6911a3e5
Merge "Clean up automated changes to requirements"