Change the sync-master command to delegate to an externally provisioned
wrapper script that controls the exact rsync command line. This script
can be provisioned in a controlled and audited manner (eg via Puppet) so
that it is guaranteed to be secure enough to allow the script to be
executed as root. Root privileges are needed to ensure that mtimes are
propagated between masters due to unix restrictions mtime modification
by non-owners of directory inodes.