Add http health-check for the api app

Also rename the 'elb' argument to 'simple' to be more descriptive.

The healthcheck is going to issue requests to `/_status?simple=true`
just to check that gunicorn is up and running - we don't need to go all
the way to the database, as this would have adverse impact when api is
under load.

manifest-api-base.yml

@@ -51,6 +51,8 @@ memory: 1G
 
 applications:
   - name: notify-api
+    health-check-type: http
+    health-check-http-endpoint: /_status?simple=true
 
   - name: notify-api-db-migration
     command: sleep infinity