Elasticsearch 5: Log transport client on stderr

This commit sets the log of the internal transport client used in the
Elasitcsearch 5 binding to use standard error instead of standard out.

elasticsearch5/src/main/resources/log4j2.properties

@@ -2,6 +2,7 @@ appender.console.type = Console
 appender.console.name = console
 appender.console.layout.type = PatternLayout
 appender.console.layout.pattern = [%d{ISO8601}][%-5p][%-25c{1.}] %marker%m%n
+appender.console.targetStr = SYSTEM_ERR
 
 rootLogger.level = info
 rootLogger.appenderRef.console.ref = console