This commit checks the validity of value change of wsrep_sst_method variable.
The validity check is same as happens in donor node when incoming SST request
is parsed.

The commit has also a mtr test: wsrep.wsrep_variables_sst_method which verifies
that wsrep_sst_method can be succesfully changed to acceptable values and that
the SET command results in error if invalid value was entered.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>