When the Galera is configured to use mariabackup, the
synchronization takes place by connecting to the
database with configuratble user credentials.
Generate a random password for this use case.
Related-Bug: #1973872
(Fixed merge conflict manually to prevent backporting
Ic7ff36a5f3f4eceeb6c8a338093e956b7db00533 with it)
Change-Id: I1509bd30fbd253790b17e04ef15dca6c58de7311
(cherry picked from commit 105a0c8627)
(cherry picked from commit a1f50b819b)
(manually fixed cherry pick backport in test plan)