Use stable branch for upper constraints

Backport 2d732ec4 inadvertently
changed the upper-constraints.txt URL to drop the reference to
the stable branch. This switched back over to the stable/pike
version of the file.

Change-Id: I2a35cd9c467545e1909df410d77ef43a756ba4bc

tox.ini

@@ -8,7 +8,7 @@ setenv =
     BRANCH_NAME=master
 install_command = pip install {opts} {packages}
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/pike}
   -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'