Add upper constraints for testing

This change adds upper constraints for running tox in order
to both solve the issue with sphinx 1.6.1 and pbr breaking
the docs gate [0], as well as helping prevent this from
occuring in the future when a tool updates and ends up
causing issue with other required libraries.

This also adds an exclusion for 1.6.1 in test requirements
as posted here [1].

[0] https://bugs.launchpad.net/pbr/+bug/1691129
[1] http://lists.openstack.org/pipermail/openstack-dev/2017-May/116821.html

Change-Id: I025e75fed186236be51c0fcfb7d35f694b0e42d8

test-requirements.txt

@@ -1,3 +1,3 @@
-sphinx>=1.5.1 # BSD
+sphinx>=1.5.1,!=1.6.1 # BSD
 oslosphinx>=4.7.0 # Apache-2.0
 pbr>=3.0.0 # Apache-2.0

tox.ini

@@ -4,7 +4,7 @@ envlist = docs
 skipsdist = True
 
 [testenv]
-install_command = pip install -U {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 passenv = *_proxy *_PROXY