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 http://lists.openstack.org/pipermail/openstack-dev/2018-March/128352.html
for more details.
Change-Id: Ia65cd41c6d0f4a4847458188cc5afddb06035a23
