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.

Credit to the equivalent Ironic patch
https://review.openstack.org/#/c/300146/

Conflicts:
    CONTRIBUTING.rst
    tox.ini

Change-Id: Ica668afedf622a0be8e8566ac88e1d0020ed9bc7
Closes-Bug: #1563038
(cherry picked from commit 16594518f3)

CONTRIBUTING.rst

@@ -52,6 +52,10 @@ interpreter of one of supported versions (currently 2.7 and 3.4), use
     Support for Python 3 is highly experimental, stay with Python 2 for the
     production environment for now.
 
+.. note::
+    Users of Fedora <= 23 will need to run "sudo dnf --releasever=24 update
+    python-virtualenv" to run unit tests
+
 There is a simple functional test that involves fetching the ramdisk from
 Github::
 

tox.ini

@@ -3,6 +3,7 @@ envlist = py27,py34,pep8,func
 
 [testenv]
 usedevelop = True
+install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/liberty} {opts} {packages}
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
@@ -14,9 +15,21 @@ setenv = PYTHONDONTWRITEBYTECODE=1
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:venv]
+# NOTE(amrith) The setting of the install_command in this location
+# is only required because currently infra does not actually
+# support constraints files for the environment job, and while
+# the environment variable UPPER_CONSTRAINTS_FILE is set, there's
+# no file there. It can be removed when infra changes this.
+install_command = pip install -U {opts} {packages}
 commands = {posargs}
 
 [testenv:releasenotes]
+# NOTE(amrith) The setting of the install_command in this location
+# is only required because currently infra does not actually
+# support constraints files for the release notes job, and while
+# the environment variable UPPER_CONSTRAINTS_FILE is set, there's
+# no file there. It can be removed when infra changes this.
+install_command = pip install -U {opts} {packages}
 envdir = {toxworkdir}/venv
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html