Cluster support for wsrep_sst_auth configuration

To make use of *xtrabackup* (or the recommend *xtrabackup-v2*) [wsrep_sst_method](http://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_sst_method), [wsrep_sst_auth](http://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_sst_auth) needs to be configured in order for SST to operate.

This is more useful in a wrapper cookbook context. 
This should be unless (instead of if !)