(#42354153) Don't define /mnt/elasticsearch twice
(#42354153) Don't define /mnt/elasticsearch twice

Remove an unrelated blocker for running multiple ES instances on the same
node. Use `defined()` to ensure that it only appears in the catalog once.

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