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: I911c66b2e9971a3e134c482a4b4ffa529d358a76
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.d/jobs.yaml

@@ -48,6 +48,7 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
+        - openstack-tox-lower-constraints
 
     gate:
       jobs:
@@ -65,6 +66,7 @@
               - ^.*\.rst$
               - ^doc/.*$
         - tripleo-buildimage-overcloud-full-centos-7
+        - openstack-tox-lower-constraints
 
 - job:
     name: dib-dsvm-functests-python2-centos-7

lower-constraints.txt

@@ -0,0 +1,49 @@
+alabaster==0.7.10
+appdirs==1.3.0
+astroid==1.6.2
+Babel==2.3.4
+coverage==4.0
+decorator==3.4.0
+docutils==0.11
+dulwich==0.15.0
+extras==1.0.0
+fixtures==3.0.0
+flake8==2.5.4
+hacking==0.10.0
+imagesize==0.7.1
+iso8601==0.1.11
+isort==4.3.4
+Jinja2==2.10
+keystoneauth1==3.4.0
+lazy-object-proxy==1.3.1
+linecache2==1.0.0
+MarkupSafe==1.0
+mccabe==0.2.1
+mock==2.0.0
+mox3==0.20.0
+networkx==1.10
+openstackdocstheme==1.18.1
+os-client-config==1.28.0
+oslotest==3.2.0
+pbr==2.0.0
+pep8==1.5.7
+pyflakes==0.8.1
+Pygments==2.2.0
+pylint==1.7.1
+python-mimeparse==1.6.0
+python-subunit==1.0.0
+pytz==2013.6
+PyYAML==3.12
+reno==2.5.0
+requests==2.14.2
+requestsexceptions==1.2.0
+six==1.10.0
+snowballstemmer==1.2.1
+Sphinx==1.6.2
+sphinxcontrib-websupport==1.0.1
+stevedore==1.20.0
+testrepository==0.0.18
+testtools==2.2.0
+traceback2==1.4.0
+unittest2==1.1.0
+wrapt==1.7.0

tox.ini

@@ -45,3 +45,10 @@ commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasen
 [flake8]
 ignore = E125,E126,E127,H202,H803
 exclude =  .venv,.git,.tox,dist,doc/build,doc/source,*lib/python*,*egg,build,conf.py
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt