Remove reliance on FACTER_govuk_platform environment variable

Background:
- This environment variable has been set by puppet but has been abused in the past to perform magic based on the environment.
- It is being replaced in Puppet by feature toggled for development-only features
- The current setting for FACTER_govuk_platform is 'production' for all environments that are not the development VM.
  - It is therefore set to exactly the same value as Rails.env

Major 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. This has not been fixed.
- `['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.

This also fixes #1481