Check cluster health in integration test wait condition

With this commit we do not check only if an endpoint is up but
we also check that the cluster status is green. Previously,
builds sporadically failed to pass this condition.

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -57,12 +57,10 @@ class ClusterConfiguration {
     @Input
     Closure waitCondition = { NodeInfo node, AntBuilder ant ->
         File tmpFile = new File(node.cwd, 'wait.success')
-        ant.echo(message: "[${LocalDateTime.now()}] Waiting for elasticsearch node ${node.httpUri()}", level: "info")
-        ant.get(src: "http://${node.httpUri()}",
+        ant.get(src: "http://${node.httpUri()}/_cluster/health?wait_for_nodes=${numNodes}",
                 dest: tmpFile.toString(),
                 ignoreerrors: true, // do not fail on error, so logging buffers can be flushed by the wait task
                 retries: 10)
-        ant.echo(message: "[${LocalDateTime.now()}] Finished waiting for elasticsearch node ${node.httpUri()}. Reachable? ${tmpFile.exists()}", level: "info")
         return tmpFile.exists()
     }