Use TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is old name and deprecated
This allows to use upper-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

Change-Id: I5a45367ad1086d3a079ab54481d1d0460298e47d

.zuul.yaml

@@ -2,4 +2,4 @@
     templates:
       - horizon-non-primary-django-jobs
       - check-requirements
-      - openstack-python3-xena-jobs
+      - openstack-python3-zed-jobs

tox.ini

@@ -35,7 +35,7 @@ commands =
 
 [testenv:docs]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
@@ -51,7 +51,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,.ropeproject,tools,hori
 
 [testenv:releasenotes]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html