Add Icinga check for Jenkins agent status
Add Icinga check for Jenkins agent status

This uses the create_resource method we use to add new Jenkins agents to
also create a basic check that will let us know if the master is unable
to connect to the specified agent.

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