Make LC_ALL default to 'en_US.UTF-8'.

The `LC_ALL` environment variable is set to `en_US` in `/etc/default/locale`
in the precise32 vagrant box, see:

https://github.com/mitchellh/vagrant/issues/1188

When running the python unit tests in the devstack machine, several of the
tests fail with a `UnicodeEncodeError` when trying to create folders with names
that contain non-ascii characters.

Explicitly setting `LC_ALL `to `en_US.UTF-8` before running the tests fixes this issue,
but it would be nice if the box was set up with the `en_US.UTF-8` locale by default.