It would be very nice if the mariabackup SST script would pass the datadir on to the move command. THis would avoid a problem like this:
[root@mariadb-4 .sst]# cat innobackup.move.log
180221 10:59:03 innobackupex: Starting the move-back operation

IMPORTANT: Please check that the move-back run completes successfully.
At the end of a successful move-back run innobackupex
prints "completed OK!".

--innobackupex based on MariaDB server 10.2.13-MariaDB Linux (x86_64)
Error: datadir must be specified.

Right now, it is mandatory to have the datadir in the configuration file.