Force LANGUAGE=en_US in test runs

If your native locale is not en_US and you run tests, some will fail due
to the message translation that happens since the assertion messages are
in en_US (not translated, just hard-coded in the tests).

I recreated the bug by setting LANGUAGE=en_GB in my tox.ini and tested
against nova.tests.unit.api.openstack.test_wsgi.

Note that setting LANG=en_US had no effect.

Closes-Bug: #1427745

Change-Id: Ib1c71d0d4170ba1a7ed2fe3eaf0e2d8071ec01d6

tox.ini

@@ -14,6 +14,7 @@ install_command = pip install -U --force-reinstall {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          OS_TEST_PATH=./nova/tests/unit
+         LANGUAGE=en_US
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -35,6 +36,7 @@ install_command = pip install -U --force-reinstall {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
        PYTHONHASHSEED=0
        OS_TEST_PATH=./nova/tests/functional
+       LANGUAGE=en_US
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =