add lower-constraints job

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: Ia0547a12c756388dc521c5eed7897140fe0dfd61
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -76,3 +76,10 @@ commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasen
 [testenv:bandit]
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r watcher -x tests -n5 -ll -s B320
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt