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: I00b41e7e3bcd86677084243917a205c703097674
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.yaml

@@ -16,6 +16,10 @@
         - sahara-image-elements-buildimages-vanilla:
             voting: false
 
+        - openstack-tox-lower-constraints
+    gate:
+      jobs:
+        - openstack-tox-lower-constraints
 - job:
     name: sahara-image-elements-buildimages-basejob
     timeout: 5400

lower-constraints.txt

@@ -0,0 +1,16 @@
+Babel==2.5.3
+bashate==0.5.1
+decorator==4.2.1
+dib-utils==0.0.11
+diskimage-builder==2.11.0
+flake8==2.5.5
+hacking==0.12.0
+mccabe==0.2.1
+networkx==1.11
+pbr==3.1.1
+pep8==1.5.7
+pyflakes==0.8.1
+pytz==2018.3
+PyYAML==3.12
+six==1.11.0
+stevedore==1.28.0

tox.ini

@@ -36,3 +36,10 @@ ignore=safe_sudo
 show-source = true
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,tools
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt