Use upper-constraints

Using upper-constraints in the keystone-specs tox environment ensures
that libraries go through validation in the requirements project and
don't break our CI.

Change-Id: Ic38c11bec5fe50c7fff7c1f4dec86504a29ba222

tox.ini

@@ -7,7 +7,8 @@ skipsdist = True
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/requirements.txt
 commands =
     stestr run --slowest {posargs}
     doc8 specs/