Set the GOVUK_APP_DOMAIN_EXTERNAL environment variable
Set the GOVUK_APP_DOMAIN_EXTERNAL environment variable

This matches up with changes in version 2.1.0 of Plek, which adds
support for explicit external URLs, and can be configured through this
environment variable.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet