Test with LANG set to UTF-8

Tox 2.0 does not pass the environment from the caller through anymore.
THis means your locale LANG setting is not set which makes the utf8
tests unhappy in JJB. Correct this by explicitly setting a LANG of
en_US.UTF-8. The important bit is the UTF-8 specification, en_US was
chosen somewhat arbitrarily.

Change-Id: I0fc5b5ab80414b392b7cebe9b4e0ebc5cc42c61c

tox.ini

@@ -11,6 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          SUBUNIT_FORMATTER=tee testr_subunit_log
          OS_STDOUT_NOCAPTURE=False
          PYTHONHASHSEED=0
+         LANG=en_US.UTF-8
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt