Allow configuring MySQL WSREP SST provider

Allow configuring MySQL WSREP SST provider

On some arm64 platforms xtrabackup is broken due to an outdated
MySQL/InnoDB code that it uses which is missing. To work around that
this patch allows choosing which WSREP SST provider to use. Aside of
already supported `xtrabackup-v2` and `mysqldump`, the `rsync` method
has been added as it is comparable to xtrabackup in terms of speed (or
is faster), has been validated to work on arm64 platforms where
xtrabackup is broken and it doesn't rely on db mechanisms to perform the
state transfer.

This patch is tied to a patch in fuel-web that introduces configuration
options for WSREP SST provider and in case nothing is specified, it
will choose `xtrabackup-v2`.

Signed-off-by: Stanislaw Kardach <stanislaw.kardach@caviumnetworks.com>