make "innodb_log_group_home_dir" an attribute

Xtrabackup-v2 SST is currently recommended if you have innodb-log-group_home-dir/innodb-data-home-dir in your cnf. Refer to sst-special-dirs for more. -- according to http://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_sst_auth
"innodb_log_group_home_dir" is an required option to use Xtrabackup-v2 SST.
Tested on Ubuntu 12.04, Percona 5.6, wsrep_sst_method = xtrabackup-v2