Add missing upper-constraints.txt to deps in docs testenv

The docs testenv in tox.ini was missing usage of upper-constraints.txt
from the global requirements project.  This meant that in some
circumstances, such as the openstack-tox-docs CI job, Python 2.7.x
would be used but Sphinx 2.x would get installed which is not
compatible with Python 2.7.x, causing tox failures like this:

  2019-05-08 12:45:48.060953 | TASK [tox : Run tox without tests]
  2019-05-08 12:45:48.759277 | ubuntu-bionic | docs create: /home/zuul/src/opendev.org/openstack/os-resource-classes/.tox/docs
  2019-05-08 12:45:52.775007 | ubuntu-bionic | docs installdeps: -r/home/zuul/src/opendev.org/openstack/os-resource-classes/doc/requirements.txt
  2019-05-08 12:45:53.838450 | ubuntu-bionic | ERROR: invocation failed (exit code 1), logfile: /home/zuul/src/opendev.org/openstack/os-resource-classes/.tox/docs/log/docs-1.log
  2019-05-08 12:45:53.838669 | ubuntu-bionic | ================================== log start ===================================
  2019-05-08 12:45:53.838995 | ubuntu-bionic | DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
  2019-05-08 12:45:53.839207 | ubuntu-bionic | Looking in indexes: http://mirror.iad.rax.openstack.org/pypi/simple, http://mirror.iad.rax.openstack.org/wheel/ubuntu-18.04-x86_64
  2019-05-08 12:45:53.839421 | ubuntu-bionic | Collecting sphinx!=1.6.6,!=1.6.7,>=1.6.2 (from -r /home/zuul/src/opendev.org/openstack/os-resource-classes/doc/requirements.txt (line 1))
  2019-05-08 12:45:53.839619 | ubuntu-bionic |   Downloading http://mirror.iad.rax.openstack.org/wheel/ubuntu-18.04-x86_64/sphinx/Sphinx-2.0.1-py2.py3-none-any.whl (3.2MB)
  2019-05-08 12:45:53.839749 | ubuntu-bionic | ERROR: Sphinx requires Python '>=3.5' but the running Python is 2.7.15
  2019-05-08 12:45:53.839778 | ubuntu-bionic |
  2019-05-08 12:45:53.839912 | ubuntu-bionic | =================================== log end ====================================
  2019-05-08 12:45:53.840347 | ubuntu-bionic | ERROR: could not install deps [-r/home/zuul/src/opendev.org/openstack/os-resource-classes/doc/requirements.txt]; v = InvocationError(u'/home/zuul/src/opendev.org/openstack/os-resource-classes/.tox/docs/bin/pip install -r/home/zuul/src/opendev.org/openstack/os-resource-classes/doc/requirements.txt', 1)

So introduce usage of upper-constraints.txt, as is already done in os-traits.

Of course we should also switch to Python 3, but even with that change
we still need to use upper-constraints.txt to prevent similar
categories of failure in the future.

Change-Id: I8f4e72bd4b34cfd177c35469e0bc7d83bee705c7

tox.ini

@@ -34,7 +34,9 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]