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: this copy from 1be35ff039d67c50866f2556ef847b048cbf3578

Change-Id: Iac2fc6fdbf9596209ab44fa2274bd66620043a49

tox.ini

@@ -10,6 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          LANGUAGE=en_US
          LC_ALL=en_US.UTF-8
+         PYTHONWARNINGS=default::DeprecationWarning
          TESTS_DIR=./ironic/tests/unit/
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}