LogstashCustomizer overrides existing root-logger level and handlers

LogstashCustomizer overrides existing root-logger level and handlers with LogstashFraction level and a single logstash-handler.

https://github.com/wildfly-swarm/wildfly-swarm/blob/2017.1.1/fractions/logstash/src/main/java/org/wildfly/swarm/logstash/runtime/LogstashCustomizer.java#L61

• LogstashFraction level should be set as logstash-handler level
• logstash-handler should be added into the existing root handlers to honor them.