Remove FACTER_govuk_platform from script/unpublished_docs.rb

This environment variable is set for puppet. We would like to remove the
environment variable. It is currently set as follows:

| Environment | Value of FACTER_govuk_production |
|-------------|----------------------------------|
| devvm       | development                      |
| preview     | production                       |
| staging     | production                       |
| production  | production                       |

I've made the following changes:

- When needing the app domain, use `GOVUK_APP_DOMAIN`
- When doing something different for development, use `Rails.env.development?`
- Removed `host_name` assignment. I don't see this used anywhere
remove FACTER_govuk_platform from lib/tasks/export.rake

Changes:
- `Whitehall::PUBLIC_HOSTS` - added one to map
  whitehall.test.alphagov.co.uk
- `export.rake`:
  - When Rails.env.production? use the MySQL slave
  - Pass Rails.env into the mappings classes
- `['mappings.rb', 'redirector_document_mappings.rb']`
  - removed references to ENV['FACTER_govuk_platform'] in favour of
    parameter `:platform` which is passed in.
    - when test, returns www.test and whitehall-admin.test
    - when production, returns www.gov.uk and whitehall-admin.production
    - I should note that these classes have NEVER worked for preview,
      because FACTER_govuk_platform==production in preview, so it would
      have already returned production URLs.
- `['mappings_test.rb', 'redirector_document_mappings_test.rb']`
  - Tests now consistently set `:platform` to 'test'
  - Tests do not rely on `ENV[]`
  - Tests consistently expect `test.alphagov.co.uk` domains

I've checked these tests manually for the production case by changing
'test' to 'production' at which point they fail correctly as they output
'www.gov.uk' as the public_host and 'whitehall-admin.production.alphagov.co.uk'
as the admin_host.