Galera MTR Tests: restore galera_autoinc_sst_xtrabackup.test to use xtrabackup SST

mysql-test/suite/galera/t/galera_autoinc_sst_xtrabackup.cnf

 !include ../galera_2nodes.cnf
 
 [mysqld]
-#wsrep_sst_method=xtrabackup-v2
-#wsrep_sst_auth="root:"
-#wsrep_debug=ON
+wsrep_sst_method=xtrabackup-v2
+wsrep_sst_auth="root:"
 
 [mysqld.1]
 wsrep_provider_options='base_port=@mysqld.1.#galera_port;gcache.size=1;pc.ignore_sb=true'

mysql-test/suite/galera/t/galera_autoinc_sst_xtrabackup.test

 #
 # Test that autoincrement works correctly while the cluster membership
-# is changing and IST takes place.
+# is changing and SST takes place.
 #
 
 --source include/big_test.inc