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: I65df598a852fdb75cebc0bb49c7aa7a0e7d7ab8c
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 +1,61 @@
+alabaster==0.7.10
+Babel==2.3.4
+certifi==2018.1.18
+chardet==3.0.4
+cliff==2.8.0
+cmd2==0.8.2
+coverage==4.0
+debtcollector==1.19.0
+docutils==0.11
+dulwich==0.15.0
+extras==1.0.0
+fixtures==3.0.0
+flake8==2.5.5
+hacking==0.12.0
+idna==2.6
+imagesize==0.7.1
+iso8601==0.1.12
+Jinja2==2.10
+keystoneauth1==3.4.0
+linecache2==1.0.0
+MarkupSafe==1.0
+mccabe==0.2.1
+mock==2.0.0
+monotonic==1.4
+msgpack-python==0.4.0
+msgpack==0.5.6
+netaddr==0.7.19
+netifaces==0.10.6
+openstackdocstheme==1.18.1
+oslo.config==5.2.0
+oslo.context==2.20.0
+oslo.i18n==3.15.3
+oslo.log==3.36.0
+oslo.serialization==2.18.0
+oslo.utils==3.36.0
+pbr==2.0.0
+pep8==1.5.7
+prettytable==0.7.2
+pyflakes==0.8.1
+Pygments==2.2.0
+pyinotify==0.9.6
+pyparsing==2.2.0
+pyperclip==1.6.0
+python-dateutil==2.7.0
+python-mimeparse==1.6.0
+python-subunit==1.0.0
+pytz==2018.3
+PyYAML==3.12
+requests==2.18.4
+rfc3986==1.1.0
+six==1.10.0
+snowballstemmer==1.2.1
+Sphinx==1.6.2
+sphinxcontrib-websupport==1.0.1
+stevedore==1.28.0
+testrepository==0.0.18
+testtools==2.2.0
+traceback2==1.4.0
+unittest2==1.1.0
+urllib3==1.22
+wrapt==1.10.11

tox.ini

@@ -34,3 +34,10 @@ import_exceptions = congressclient.i18n
 show-source = True
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt