Minor modifications

- Simplified test cases in wsrep.variables
- Fixed a condition in wsrep_check_opts.cc
- Fixed an "unbound variable" in wsrep_sst_rsync
MDEV-6833: SIGSEGV on shutdown with non-default wsrep_slave_threads

thd->variables' table_plugin & tmp_table_plugin should be set to
NULL for wsrep system threads.
Also made a minor change to skip checking of wsrep options if
wsrep_on is not set.