Most of the mtr tests in the galera_3nodes suite fail
for a variety of reasons with a variety of errors.

This patch fixes several substantial flaws
in the galera_3nodes suite tests and in the mtr framework
service files, adapting the tests from galera_3nodes
for the current version of MariaDB.

This patch also synchronizes some galera_3nodes-related
files with the latest changes made for MDEV-17835 (v2 patch)
and for MDEV-18379 in other branches (10.2 and 10.3).

Closes #1161