Force LANGUAGE=en_US in test runs

Basically copying commit bf6d966756d3745d2facd5ac39444646dcfae6de from
the ironic project.

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: I812a296ea6c9be303f40346c6db06673fefda8d5

tox.ini

@@ -5,6 +5,7 @@ skipsdist = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
+         LANGUAGE=en_US
 install_command = pip install -U {opts} {packages}
 deps =
     -r{toxinidir}/requirements.txt
@@ -22,6 +23,7 @@ commands =
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}
+         LANGUAGE=en_US
 commands =
     python setup.py testr --coverage {posargs}
 
@@ -30,6 +32,7 @@ commands = {posargs}
 
 [testenv:functional]
 setenv = OS_TEST_PATH=./ironicclient/tests/functional
+         LANGUAGE=en_US
 
 [flake8]
 ignore =