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.

Note: https://review.openstack.org/#/c/379581/
https://review.openstack.org/#/c/353154/

Change-Id: Ibd28c98120db0513a3ef82dcee194f3c20ebc6d3

tox.ini

@@ -10,6 +10,7 @@ setenv =
    VIRTUAL_ENV={envdir}
    BRANCH_NAME=master
    CLIENT_NAME=python-karborclient
+   PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'