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.
See [1] for details.
1. http://lists.openstack.org/pipermail/openstack-dev/2018-March/128352.html
Change-Id: I0f8b1bd44cd2f401593ce079cdecce5020807051
