Bug fix - Find the latest RHEL5 Kickstart tree path

Changed the behaviour how to find latest kickstart tree path, it affects mainly RHEL5.
With the introduction of RHEL5.7 the name of the kickstart tree directory changed from "ks-rhel-x86_64-server-5-u6" to "ks-rhel-x86_64-server-5-5.7".