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: I7042dcfb2d561819fd8098f53036aa6cc045dc56
Closes-bug: #1691431

tox.ini

@@ -5,7 +5,7 @@ envlist = 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}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -16,7 +16,7 @@ commands =
 downloadcache = ~/cache/pip
 
 [testenv:pep8]
-deps = hacking==0.10.2
+deps = hacking
 commands =
   flake8 {posargs:fuel_agent}
   flake8 {posargs:contrib/fuel_bootstrap/fuel_bootstrap_cli/fuel_bootstrap}