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.

Depends-On: Ibeabf474194b2d38273d76117a3a2f3bee5f7286
Change-Id: Iab8171ac0943c774be0d2117ac442f2e153b3c7f

tox.ini

@@ -9,6 +9,7 @@ setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C
+         PYTHONWARNINGS=default::DeprecationWarning
 usedevelop = True
 install_command =
     {toxinidir}/tools/tox_install.sh {env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}