Only set up default locale on FixMyStreet install.

FixMyStreet uses the default locale in order to have working URIs with
floating point numbers. This could be moved to the FixMyStreet install
script with its next release and then removed from here.
Installation of the `locales` package runs `locale-gen` postinst, so any locale chosen at server set up is already available. I'm not sure it feels right for an install script to install every possible locale, just in case? Vagrant's `puppetlabs/debian-7.4-64-nocm`, `puppetlabs/debian-6.0.9-64-nocm`, and `ubuntu/precise64` all have a default of `en_US.UTF-8`, for what it's worth. If the default is None, then yes, you need to make sure the locale passed by SSH is installed on the server in order for it to work. Perhaps this script should instead do the following:
* Check what locale has come over SSH - if it's UTF-8, make sure that it is installed and that the server has no default, and then continue;
* Otherwise, check if the server has a default, and that it's UTF-8, and if so continue;
* Otherwise, set up `en_GB.UTF-8` as now so that installation will proceed smoothly (e.g. in mapit's case, install_postgis requires a UTF-8 locale).
Does that work?