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: I8835451b431d29ff2a4d8eb965e8684c4499e9e8
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 +1,31 @@
+alabaster==0.7.10
+Babel==2.5.3
+bashate==0.5.1
+certifi==2018.1.18
+chardet==3.0.4
+decorator==4.2.1
+diskimage-builder==1.1.2
+docutils==0.11
+dulwich==0.19.0
+flake8==2.5.5
+hacking==0.12.0
+idna==2.6
+imagesize==1.0.0
+Jinja2==2.10
+MarkupSafe==1.0
+mccabe==0.2.1
+networkx==1.11
+openstackdocstheme==1.18.1
+pbr==2.0.0
+pep8==1.5.7
+pyflakes==0.8.1
+Pygments==2.2.0
+pytz==2018.3
+PyYAML==3.12
+requests==2.18.4
+six==1.11.0
+snowballstemmer==1.2.1
+Sphinx==1.6.2
+sphinxcontrib-websupport==1.0.1
+stevedore==1.28.0
+urllib3==1.22

tox.ini

@@ -39,3 +39,10 @@ commands = {toxinidir}/tools/docker_builder.sh
 show-source = true
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,tools
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt