Only run one type of app healthcheck when $json_health_check is set
Only run one type of app healthcheck when $json_health_check is set

From the Trello card, specific to `email-alert-api`:
- This icinga alert regularly makes a GET request to the `/healthcheck`
  endpoint in `email-alert-api` and checks that the app responds
  successfully.
- We don't need it as we already have the [`email-alert-api app
  healthcheck not ok` icinga
  alert](https://alert.publishing.service.gov.uk/cgi-bin/icinga/extinfo.cgi?type=2&host=email-alert-api-1.backend.publishing.service.gov.uk&service=email-alert-api+app+healthcheck+not+ok)
  that regularly makes GET requests to the same endpoint and if the
  response isn't successful, it fails.
- Having both is redundant because whenever `email-alert-api app
  healthcheck` fails, `email-alert-api app healthcheck not ok` fails too.

However:

- It was non-trivial to scope this to just `email-alert-api` when the
  same problem exists for all apps with /healthcheck routes.
- This change makes it so that [if apps have `$json_health_check` set to
  true](https://github.com/alphagov/govuk-puppet/search?q=json_health_check&unscoped_q=json_health_check),
  we'll only see the "healthcheck not ok" check. For apps that don't
  (the ones not in the above list), it will still run the "app_name app
  healthcheck" check, so we won't get duplicate-ish checks.

https://trello.com/c/ZDAY7kMs/1479-3-remove-the-redundant-email-alert-api-app-healthcheck-icinga-alert

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