Fix healthcheck in ES compose service (elastic#20511)

Python is not included anymore on Elasticsearch images, change the
healthcheck to be based on curl and the easier to parse cat API.

x-pack/libbeat/docker-compose.yml

@@ -29,7 +29,7 @@ services:
       file: ${ES_BEATS}/testing/environments/${TESTING_ENVIRONMENT}.yml
       service: elasticsearch
     healthcheck:
-      test: ["CMD-SHELL", 'python -c ''import urllib, json; response = urllib.urlopen("http://myelastic:changeme@localhost:9200/_cluster/health"); data = json.loads(response.read()); exit(1) if data["status"] != "green" else exit(0);''']
+      test: ["CMD-SHELL", "curl http://myelastic:changeme@localhost:9200/_cat/health?h=status | grep -q green"]
       retries: 1200
       interval: 5s
       start_period: 60s