Added new enum variable `wsrep_mode` which can be used to turn on WSREP
features which are not part of default behaviour.
Added enum `BINLOG_ROW_FORMAT_ONLY`, `REQUIRED_PRIMARY_KEY` and
`STRICT_REPLICATION`. `wsrep-mode=STRICT_REPLICATION` behaves
like variable `wsrep_strict_ddl`.

Variable wsrep_strict_ddl is deprecated and if set we use
new wsrep_mode setting instead.

Reviewed and improved by: Jan Lindström <jan.lindstrom@mariadb.com>