Allow Site.create to specify :redirector or :bouncer

* Defaults to :bouncer
Update rake task for environmental variable SITE_TYPE

* Governs what type of site gets generated:
    * data/sites/name.yml for redirector
    * data/transition-sites/name.yml for bouncer
Be accurate about location of new bouncer files
Don't create tests/mappings for bouncer sites
Merge pull request #489 from alphagov/bouncer-rake

Modify `new_site` to accept a SITE_TYPE parameter