Add constraint target to tox.ini and remove 1 dep

This adds a pip install command to tox.ini that is only used when the
tox env is passed with the 'constraints' factor appended onto it.
As such this will not effect developer workflows or current unit tests.

The initial use of this will be in a non-voting job, to verify that the
constrained checks with tox are stable.  DevStack is already running
constrained jobs, as such problems are no expected.

To run a tox with pip using constraints on a developer system a
developer should run the desired tox environment with -constraints.
For example: $(tox -epy27-constraints)
Pip will pull the current version of the upper-constraints.txt file down
from the git.openstack.org, however this method can be overriden to use
a local file setting the environment variable "UPPER_CONSTRAINTS_FILE"
to the local path or a different URL, it is passed directly to pip.

This is currently not enabled in the default tox run, however it is
possible to enable it as a default by adding it to 'envlist' in tox.ini

This also removes requirements.txt from tox.ini deps
This is redundant, per lifeless email:
http://lists.openstack.org/pipermail/openstack-dev/2015-July/069663.html

Change-Id: Id953a99cf2f83533a38a03f9934f4ec818bec03f

tox.ini

@@ -1,15 +1,16 @@
 [tox]
-minversion = 1.6
+minversion = 1.8
 envlist = py34,py27,pep8
 skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command =
+                  constraints: pip install -U --force-reinstall -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+                  pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]