When deploying a HA overcloud, allow the SST method to be
configured, to use either rsync (default) or mariabackup.
Tested by deploying a HA overcloud with mariabackup SST
(IPv4, IPv6, TLS-e) as well as with regular rsync SST
(IPv4, IPv6, TLS-e).
Note that this initial support doesn't allow SST method
to be updated via a stack update, as mariabackup requires
a dedicated MySQL user, and there is currently no
orchestration available via a minor update either.
Closes-Bug: #1973872
Depends-On: I1509bd30fbd253790b17e04ef15dca6c58de7311
Depends-On: I4d5983ada52a58cdd853bd7b701bb11eeb88be8d
Change-Id: I6b797141800ae7f1773e9a06b232dacd158732f5
4cceeebc86