Open Stack

To facilitate upper-constraints on developer systems we have a
hard-coded URL in projects tox.ini.  This URL needs to change when
after the openstack/requirements repo has created a branch for the
stable release.

This is in reference to [0].  There was some mention of possibly
adding this to stable-branch creation procedure, but as requirements
tends to release later, and that it would require a whole bunch of
special notes this seems sub-optimal.

Any thoughts on the best way to support this without a bunch of manual work?

[0]: https://review.openstack.org/#/c/267941/

Cheers,
Sachi