Adding new whitehall test for non-auth http

Previously, all http was authenticated by default, even
www.*.alphagov.co.uk - and this is desirable in preview only. A
new whitehall-only case has been implemented to explicitly ensure
AUTH_USERNAME and AUTH_PASSWORD are uninstantiated.
setting env var dependencies

Rather than defaulting to `govuk_setenv`, the standard so far is to
explicitly export the required variables. This also avoids the need to
roll out scripts on jenkins boxes
allowing for custom rake tasks

Currently only two options available. I've opened it up to pass
via cmdline