[KARAF-2270] Remove setting the JAVA_HOME in karaf-service script (to avoid to override the one defined in karaf-wrapper.conf) and update the documentation

git-svn-id: https://svn.apache.org/repos/asf/karaf/branches/karaf-2.3.x@1518491 13f79535-47bb-0310-9956-ffa450edef68