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: I6183925f1acc8bb020caa099c60b6baee61e579a
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 +1,59 @@
+alabaster==0.7.10
+appdirs==1.3.0
+Babel==2.3.4
+bandit==1.4.0
+coverage==4.0
+debtcollector==1.2.0
+docutils==0.11
+dulwich==0.15.0
+eventlet==0.18.2
+extras==1.0.0
+fasteners==0.7.0
+fixtures==3.0.0
+flake8==2.5.5
+gitdb==0.6.4
+GitPython==1.0.1
+greenlet==0.4.10
+hacking==0.12.0
+imagesize==0.7.1
+iso8601==0.1.11
+Jinja2==2.10
+keystoneauth1==3.4.0
+linecache2==1.0.0
+MarkupSafe==1.0
+mccabe==0.2.1
+mock==2.0.0
+monotonic==0.6
+mox3==0.20.0
+netaddr==0.7.18
+netifaces==0.10.4
+openstackdocstheme==1.18.1
+os-client-config==1.28.0
+oslo.config==5.2.0
+oslo.i18n==3.15.3
+oslo.utils==3.33.0
+oslotest==3.2.0
+pbr==2.0.0
+pep8==1.5.7
+pyflakes==0.8.1
+Pygments==2.2.0
+pyparsing==2.1.0
+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
+rfc3986==0.3.1
+six==1.10.0
+smmap==0.9.0
+snowballstemmer==1.2.1
+Sphinx==1.6.5
+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

@@ -49,3 +49,10 @@ commands = pip-missing-reqs -d --ignore-module=oslo_concurrency* --ignore-file=o
 
 [testenv:releasenotes]
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt