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: I2115c1c13e609c3f720c44f38f5562cead456373
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.yaml

@@ -15,6 +15,8 @@
     check:
       jobs:
         - build-i18n-docs
+        - openstack-tox-lower-constraints
     gate:
       jobs:
         - build-i18n-docs
+        - openstack-tox-lower-constraints

lower-constraints.txt

@@ -0,0 +1,48 @@
+alabaster==0.7.10
+Babel==2.5.3
+beautifulsoup4==4.6.0
+certifi==2018.1.18
+chardet==3.0.4
+debtcollector==1.19.0
+doc8==0.8.0
+docutils==0.14
+dulwich==0.19.0
+flake8==2.2.4
+hacking==0.10.3
+idna==2.6
+imagesize==1.0.0
+iso8601==0.1.12
+Jinja2==2.10
+lxml==4.1.1
+MarkupSafe==1.0
+mccabe==0.2.1
+monotonic==1.4
+msgpack==0.5.6
+netaddr==0.7.19
+netifaces==0.10.6
+openstackdocstheme==1.20.0
+oslo.config==5.2.0
+oslo.context==2.20.0
+oslo.i18n==3.20.0
+oslo.log==3.37.0
+oslo.serialization==2.25.0
+oslo.utils==3.36.0
+pbr==3.1.1
+pep8==1.5.7
+pyflakes==0.8.1
+Pygments==2.2.0
+pyinotify==0.9.6
+pyparsing==2.2.0
+python-dateutil==2.7.0
+pytz==2018.3
+PyYAML==3.12
+requests==2.18.4
+restructuredtext-lint==1.1.3
+rfc3986==1.1.0
+six==1.11.0
+snowballstemmer==1.2.1
+Sphinx==1.6.5
+sphinxcontrib-websupport==1.0.1
+stevedore==1.28.0
+urllib3==1.22
+wrapt==1.10.11

tox.ini

@@ -50,3 +50,10 @@ max-line-length = 80
 # D000: Check RST validity
 # - cannot handle "none" for code-block directive
 ignore = D000
+
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt