pin lower-constraints to run against python 2.7

Change-Id: I5c856012a04ef8f910c28b0248ea7bcda47e4816
(cherry picked from commit 281ffab2a5)

.zuul.yaml

@@ -372,6 +372,8 @@
     name: swift-tox-lower-constraints
     parent: openstack-tox-lower-constraints
     vars:
+      bindep_profile: test py27
+      python_version: 2.7
       tox_environment:
         TMPDIR: '{{ ansible_env.HOME }}/xfstmp'
 

tox.ini

@@ -145,6 +145,7 @@ commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasen
 # FIXME(dhellmann): We'll want to uncomment this
 # when the full test suite works with python 3.
 # basepython = python3
+basepython = python2.7
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt