Add lower-constraints job

create a tox environment for running the unit tests against the lower
bounds of the dependencies.

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: Iae676c4bbd00836cc6dce0f083f7aa308bbfc372

Author: pengyuesheng

.zuul.yaml

@@ -40,6 +40,7 @@
 
 - project:
     templates:
+      - openstack-lower-constraints-jobs
       - check-requirements
       - openstack-cover-jobs
       - openstack-python-jobs

tox.ini

@@ -62,3 +62,10 @@ deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -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