When the TC merged I2637dd714cbb6d38ef8b8dc1083e359207118284 we're
supposed to invoke stestr rather than testr so lets do that
Create a tox environment for running the unit tests against the lower
bounds of the dependencies.
Create a lower-constraints.txt to be used to enforce the lower bounds
in those tests.
Add openstack-tox-lower-constraints job to the zuul configuration.
for more details.
Signed-off-by: Doug Hellmann <firstname.lastname@example.org>