Enable DeprecationWarning in test environments

Many deprecations are triggered early (on imports, for example).
To make sure all DeprecationWarning messages are emitted we enable
them via the PYTHONWARNINGS environment variable.

Change-Id: Ib394e5f3f4f600cb2595294c5582695d3acf9b57

tox.ini

@@ -5,6 +5,7 @@ envlist = py34,py27,pep8
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
+         PYTHONWARNINGS=default::DeprecationWarning
 usedevelop = True
 install_command = pip install {opts} {packages}