Update TOX_CONSTRAINTS_FILE for stable/ussuri

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/ussuri branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I3305812eb3a8d7c81f66777896d243e02238031a

tox.ini

@@ -14,7 +14,7 @@ setenv =
     VIRTUAL_ENV={envdir}
     DISCOVER_DIRECTORY=sahara_plugin_mapr/tests/unit
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
@@ -30,7 +30,7 @@ commands = oslo_debug_helper -t sahara_plugin_mapr/tests/unit {posargs}
 
 [testenv:pep8]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/doc/requirements.txt
@@ -45,7 +45,7 @@ commands = {posargs}
 
 [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/ussuri}
   -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf doc/build/html
@@ -65,7 +65,7 @@ whitelist_externals =
 
 [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/ussuri}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build releasenotes/html