Add a SELINUX relabel function for RHEL-like oses

This was commited by `Nikos Skalkotos <skalkoto@gmail.com>` on 05-2011 with the following comment:

Forced filesystem relabeling in the initial boot of a Fedora image. This is needed by SELinux in recent Fedora releases, because in any other case, SELinux will consider files that were externally altered by gnt-instance-image scripts as being in a inconsistent state and will not permit the system to use them, causing a denial of service.