Fetch upper constraints from opendev.org

the imagebuild/common/generate_upper_constraints.sh
is currently failing as the curl command in this file does not
handle redirects.

Instead of allowing redirects, use the proper current link to the
upper constraints file.

Change-Id: I92c5bfa65ba149687f61172c2a89ff38660a58cf

tox.ini

@@ -5,7 +5,7 @@ envlist = py3,py27,functional,pep8
 
 [testenv]
 usedevelop = True
-install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   PYTHONDONTWRITEBYTECODE = 1