Harden envvars
Harden envvars

We will potentially be using a user that has access to another
environments S3 bucket, so we should ensure that only a specific user
has access to the environmental variables.

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