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: I6aa7ffba825c9186198f691992199f51f8804823
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
diff --git a/tox.ini b/tox.ini
index d5e3b91..3c2c8ad 100644
--- a/tox.ini
+++ b/tox.ini
@@ -78,3 +78,10 @@
 
 [hacking]
 local-check-factory = patrole_tempest_plugin.hacking.checks.factory
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt