Donor selection from SST

Currently, in SST script, no matter what is done, the donor
cannot be changed later on.

There are times when the selected donor may look infeasible at SST
time, possibly from long running transactions which can delay
FTWRL etc.

So, we need to add galera interaction for exit codes like EBUSY
(which exist now in some SST scripts but may not work fully),
wherein the current donor can be skipped and new donor selected.

Possibility for other user cases as well.

This is related to https://blueprints.launchpad.net/percona-xtradb-cluster/+spec/trigger-timeout-of-ftwrl-on-sst