* Made galera_3nodes.galera_ipv6_mariabackup deterministic with respect
to donor selection, fixed assert grep definitions to match
Galera 4 behavior and recorded.
* Removed extra connection setup from galera_3nodes.galera_ipv6_mysqldump
and recorded.
The second line of changes related to replacing xtrabackup with
mariabackup:
1) All unnecessary references to xtrabackup are removed from
the documentation, from some comments, from the control files
that are used to prepare the packages.
2) Made corrections of the tests from the galera_3nodes suite
that mentioned xtrabackup or the old (associated with xtrabackup)
version of innobackupex.
3) Fixed flaws in the galera_3nodes mtr suite control scripts,
because of which they could not work with mariabackup.
4) Fixed numerous bugs in the SST scripts and in the mtr test
files (galera_3nodes mtr suite) that prevented the use of Galera
with IPv6 addresses.
5) Fixed flaws in tests for rsync and mysqldump (for galera_3nodes
mtr tests suite). These tests were not performed successfully without
these fixes.
https://jira.mariadb.org/browse/MDEV-17835
