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: I0906638c2fd77dbdcff56ffc43e5c3b70ca729f5
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.yaml

@@ -0,0 +1,7 @@
+- project:
+    check:
+      jobs:
+        - openstack-tox-lower-constraints
+    gate:
+      jobs:
+        - openstack-tox-lower-constraints

lower-constraints.txt

@@ -0,0 +1,30 @@
+alabaster==0.7.10
+Babel==2.3.4
+coverage==4.0
+docutils==0.11
+dulwich==0.15.0
+extras==1.0.0
+fixtures==3.0.0
+flake8==2.5.5
+imagesize==0.7.1
+Jinja2==2.10
+linecache2==1.0.0
+MarkupSafe==1.0
+mccabe==0.4.0
+openstackdocstheme==1.18.1
+pbr==2.0.0
+pep8==1.7.1
+pyflakes==1.0.0
+Pygments==2.2.0
+python-mimeparse==1.6.0
+python-subunit==1.0.0
+pytz==2013.6
+requests==2.14.2
+six==1.10.0
+snowballstemmer==1.2.1
+Sphinx==1.6.5
+sphinxcontrib-websupport==1.0.1
+testrepository==0.0.18
+testtools==2.2.0
+traceback2==1.4.0
+unittest2==1.1.0

tox.ini

@@ -28,3 +28,10 @@ commands =
 show-source = true
 ignore = E721
 exclude=.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt