In galera_3nodes.galera_safe_to_bootstrap node restart can happen too soon, when earlier SST joiner process is still active in the node.
Similar issue may hurt other mtr tests as well.

This is second variant of fix for this issue. Here we only change rsync SST script to wait a little bit if lingering SST rsync is observed to be in execution.
We assume that the previous mysqld and SST processes have been already signaled to abort during earlier stataup attempt.

If other SST methods (than rsync) suffer from similar overlapping SST execution, they should be sorted out separately within each SST method handler scripts.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>