Use upper constraints

This commit is used to exclude new python packages from being
installed during tox tests and protect our CI from sporadical
failures caused by new packages.

NOTE. I have used mitaka constraints intentionally, since we
need newere packages than specified in liberty's
upper-constraints.

Change-Id: Ide77400b8d77631eaafa8cbea0679a23104d1aed
Closes-bug: #1705310

tox.ini

@@ -6,7 +6,7 @@ envlist = py26,py27,py34,pep8
 
 [testenv]
 usedevelop = True
-install_command = pip install --allow-external -U {opts} {packages}
+install_command = pip install -c https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/mitaka {opts} {packages}
 whitelist_externals = bash
                       python
 changedir={toxinidir}/nailgun
@@ -36,7 +36,7 @@ deps = -r{toxinidir}/nailgun/requirements.txt
 commands = {posargs:}
 
 [testenv:pep8]
-deps = hacking==0.10
+deps = hacking
 usedevelop = False
 commands =
     flake8 {posargs:nailgun}