Force en_US language for tests

This patch forces the use of the en_US locale for tests run.
This is done to avoid tests failing due to message translation.
For example:

When asserting if the message of an exception is correct, those are
usually hardcoded in the tests.

Change-Id: Ia8068b454f1461f17994733afc57337f4b017460

tox.ini

@@ -9,6 +9,8 @@ install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.opens
 setenv =
   VIRTUAL_ENV={envdir}
   OS_TEST_PATH=./ironic_python_agent/tests/unit
+  LANGUAGE=en_US
+  LC_ALL=en_US.utf-8
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}