Cherry-pick elastic#10499 to 6.x: Updating stack versions to latest i…

…n docker images used for tests (elastic#10502)

Cherry-pick of PR elastic#10499 to 6.x branch. Original message: 

Updates Elastic stack docker images being used in tests to the latest released version, 6.6.0.

metricbeat/module/elasticsearch/_meta/Dockerfile

@@ -1,2 +1,2 @@
-FROM docker.elastic.co/elasticsearch/elasticsearch:6.5.1
+FROM docker.elastic.co/elasticsearch/elasticsearch:6.6.0
 HEALTHCHECK --interval=1s --retries=300 CMD curl -f http://localhost:9200/_xpack/license

metricbeat/module/kibana/_meta/Dockerfile

@@ -1,2 +1,2 @@
-FROM docker.elastic.co/kibana/kibana:6.5.1
+FROM docker.elastic.co/kibana/kibana:6.6.0
 HEALTHCHECK --interval=1s --retries=300 CMD curl -f http://localhost:5601/api/status | grep '"disconnects"'

metricbeat/module/logstash/_meta/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.elastic.co/logstash/logstash:6.3.0
+FROM docker.elastic.co/logstash/logstash:6.6.0
 
 COPY healthcheck.sh /
 ENV XPACK_MONITORING_ENABLED=FALSE

testing/environments/latest.yml

@@ -3,7 +3,7 @@
 version: '2.3'
 services:
   elasticsearch:
-    image: docker.elastic.co/elasticsearch/elasticsearch:6.4.0
+    image: docker.elastic.co/elasticsearch/elasticsearch:6.6.0
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:9200"]
       retries: 300
@@ -16,7 +16,7 @@ services:
       - "xpack.security.enabled=false"
 
   logstash:
-    image: docker.elastic.co/logstash/logstash:6.4.0
+    image: docker.elastic.co/logstash/logstash:6.6.0
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:9600/_node/stats"]
       retries: 300
@@ -26,7 +26,7 @@ services:
       - ./docker/logstash/pki:/etc/pki:ro
 
   kibana:
-    image: docker.elastic.co/kibana/kibana:6.4.0
+    image: docker.elastic.co/kibana/kibana:6.6.0
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:5601"]
       retries: 300