Use upper-constraints for py27 env

py27 jobs are failing due to incompatible python
dependencies, use upper constraints to fix it.

Use constraints for last py2 release i.e train.

Also use stable/train branch for openstack/requirements.

Change-Id: Idf344c5bb3db63fc3afde56d55bbdb1b8b048278

.zuul.yaml

@@ -1,3 +1,13 @@
 - project:
     templates:
       - openstack-python-jobs
+    check:
+      jobs:
+        - openstack-tox-py27:
+            required-projects: &project_overrides
+              - name: opendev.org/openstack/requirements
+                override-checkout: stable/train
+    gate:
+      jobs:
+        - openstack-tox-py27:
+            required-projects: *project_overrides

tox.ini

@@ -10,6 +10,10 @@ deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/train}
+       {[testenv]deps}
+
 [testenv:venv]
 commands = {posargs}