add upper_constraints support

- This change enforces the use of upper_constraints
  in tox envs.

NOTE(ralonsoh): Conflicts are because we're missing a number
  of changes, none of which are necessary to include here. In
  addition, the constraints URL is modified to ensure constraints
  for this stable version are used.

Change-Id: I2badb5a3cd5f1ab2a66d3ebd97432b77985ada1b
(cherry picked from commit 96b5a11d35)

tox.ini

@@ -7,9 +7,12 @@ skipsdist = True
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
-   VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+  VIRTUAL_ENV={envdir}
+  CONSTRAINTS_OPT=-c {env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/queens}
+deps =
+  {env:CONSTRAINTS_OPT}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash
 
 [tox:jenkins]