donor uses invalid SST methods

during SST a joiner sends an sst method name to the donor. Donor then appends it to the "wsrep_sst_" string to get the name of the sst script to use, e.g. wsrep_sst_rsync. There is no validation or filtering here, so if the malicious joiner sends, for example, "rsync `rm -rf /`" the donor will execute that too.