wsrep_sst_xtrabackup should also support two stage SST/IST like percona

in wsrep_sst_xtrabackup with mariadb-galera mentioned a bug lp:1193240, which need manual clean up the data dir for SST, so this makes xtrabackup method will always need manual operation to use. but latest developement of percona-xtradb-cluster sovled this problem by making this script a two stage SST, which means if need SST, then script will clean the data files for SST, if need IST, then script operates like current working way.

the fix bug is lp:1222122 , as you can see, this is a high importance bug in percona.

so with this two stage xtrabackup SST, the program will handle the different situation. and make this SST really useful.

So I hope mariadb-galera can also have this function.