Change crazy wsrep_sst_donor behaviour

Hi,

Currently there is a small note in the documentation for wsrep_sst_donor:
'If no suitable donor is still found, the rest of the donor nodes are checked for suitability only if the donor list has a "terminating-comma"'

This means, if you have wsrep_sst_donor set to your preferred donor, the node will not be able to rejoin of that node is down. This is, in my opinion, crazy and dangerous.

Please move this behaviour to a different configuration parameter, let's say 'wsrep_sst_fallback' which defaults to 1.

If you do not agree this behaviour should be changed we should at least change the documentation.