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: I6a7a78800caf3c536603ae0bcfeb03830c8a5860
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -55,3 +55,10 @@ exclude = .venv*,.tox,dist,*egg,build,.git,doc,*lib/python*,.update-venv
 
 [hacking]
 import_exceptions = six.moves,glanceclient._i18n
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt