wsrep_sst_mariabackup does not distinguish between VERIFY_CA and VERIFY_IDENTITY

The `wsrep_sst_rsync` and `wsrep_sst_mariabackup` scripts accept values `VERIFY_CA` and `VERIFY_SST` options for the `[sst] ssl-mode` setting.

With the rsync script both lead to different TLS settings, but the mariabackup script does not actually distinguish between the two and just checks for

"ssl-mode starts with VERIFY*"

so that the behavior will actually be exactly the same for both distinct VERIFY options.