[KARAF-2568] Backport of JMX Security itest to 2.x

Also contains fixes to the users.properties file and additional testing to ensure that it is not possible to log in as a group directly.



git-svn-id: https://svn.apache.org/repos/asf/karaf/branches/karaf-2.x@1545567 13f79535-47bb-0310-9956-ffa450edef68