sst mariabackup required datadir in my.cnf

when we switched from rsync sst method to mariabackup and forced an sst the sst failed with the following error message, which is not easy to see/find:

[root@notebook31 .sst]# tail innobackup.move.log
180322 17:37:22 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.

Maybe it is possible to write down datadir automatically in my.cnf when mariadb is rolled out via RPM?