Merge branch 'mysql-5.6-cluster-7.4' into mysql-5.7-cluster-7.5
Adding check to stop Ndb slave if multi-threaded slave configuration or if multi-master setup is detected.
Added new test case for checking unsupported slave configurations.
Added new test case for checking if multi-master replication is setup.
Created generic 3 cluster config to be used by ndb_rpl_slave_multi_source_config.test and ndb_rpl_break_3_chain.test