Add function to list DR nodes
Add function to list DR nodes

This commit adds a flag so you are able to list if a DR node identifies
itself as being such, and a flag so it only lists nodes that are NOT in
the DR environment.

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