Make tox respect upper-constraints.txt

This will force pip install to use the upper-constraints.txt specified
version of pip modules. When you don't do this, you are out on the
bleeding edge and become unstable everytime some python library in the
wide world changes in a way that you don't expect.

Post jobs are not yet compatible with the upper-constraints file, so
override the install_command there to skip upper-constraints.

Blatantly cribbed from the equivalent trove patch:
https://review.openstack.org/#/c/298850

Change-Id: I73aa8cf37adc36db2c56a8f29ea165775a83dc7f
Closes-Bug: #1563038
(cherry picked from commit e27e7ed)
(cherry picked from commit 6e775b8)

tox.ini

@@ -5,9 +5,7 @@ envlist = py34,py27,pep8
 
 [testenv]
 usedevelop = True
-install_command =
-                  constraints: pip install -U --force-reinstall -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/mitaka} {opts} {packages}
-                  pip install -U {opts} {packages}
+install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/mitaka} {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          LANGUAGE=en_US