Set asset manager plek override for Smokey
Set asset manager plek override for Smokey

Smokey tests in AWS staging and production are failing because they are
trying to access Asset Manager over an internal URI: https://asset-manager.production.govuk-internal.digital/assets/513a0efbed915d425e000002

This sets an environment variable that overrides Plek with the URI for
asset manager.

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