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: I0a698c07a3052c52cf0c112c31f1caf33f3df972

tox.ini

@@ -8,6 +8,7 @@ usedevelop = True
 install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
+   PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'