Force LANGUAGE=en_US in test runs

Basically back-porting what was done for Nova in this patch:
https://review.openstack.org/#/c/160892/

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

Note that setting LANG=en_US had no effect.

Change-Id: I16fab554612a718871625f688dd5501a69fb3417

tox.ini

@@ -8,6 +8,7 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
+         LANGUAGE=en_US
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash
@@ -25,6 +26,7 @@ commands =
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}
+         LANGUAGE=en_US
 commands =
   python setup.py testr --coverage {posargs}
 
@@ -48,6 +50,7 @@ commands =
 
 [testenv:venv]
 setenv = PYTHONHASHSEED=0
+         LANGUAGE=en_US
 commands = {posargs}
 
 [flake8]