Xtrabackup SST Setup and Validate script

I propose we create a script that is distributed with the PXC server package that will setup and validate SST for xtrabackup based on the wsrep_sst_auth setting -- the big idea here is if wsrep_sst_auth is set, this script (with the proper credentials) can go check the local node to see if the GRANT is needed, the right privileges are granted, and fix the GRANT if needed. It should then emit output indicating that SST should be ready to go.

There may be other checks it could sanely do, perhaps around some of the new SST features.