Update TOX/UPPER_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: I220563d4d84fec1fba501cd80887b03e43b4c020

tox.ini

@@ -20,7 +20,7 @@ setenv =
 
 install_command = pip install {opts} {packages}
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
@@ -51,7 +51,7 @@ commands =
 
 [testenv:pylint]
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
        -r{toxinidir}/requirements.txt
        pylint==0.26.0
 commands = bash tools/lintstack.sh
@@ -73,7 +73,7 @@ commands =
 
 [testenv:docs]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build doc/source/contributor/api/ .autogenerated