Enable DeprecationWarning in test environments

Many deprecations are triggered early (on imports, for example)
before the warnings are enabled by the WarningsFixture in the
base test class.

To make sure all DeprecationWarning messages are emitted we enable
them via the PYTHONWARNINGS environment variable.

Change-Id: Ica6397a04efdb089b2ae631906886b3ac44862f5

tox.ini

@@ -11,6 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_OPENSTACK_SHOW_ELAPSED=1
          NOSE_OPENSTACK_STDOUT=1
          NO_EVENTLET=1
+         PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = nosetests --exclude=mysql {posargs}