Env variable that shows preview, staging or prod
Env variable that shows preview, staging or prod

GOVUK_ENV is a rails style development/production flip but we want to
know whether we are in preview, staging or production. I have added a
new env varaible that will contain this information.

alphagov/pp-puppet,alphagov/pp-puppet,alphagov/pp-puppet,alphagov/pp-puppet