Add option to specify master connection for MariaDB multi-source replication to repl check plugins

show slave 'slave1' status - output for master connection "slave1".
show all slaves status - output for all the masters connections, 1 row per each.
show slave status - empty or output for default connection if configured.

Here is how we can set default connection on session level:
set default_master_connection='slave1';

https://mariadb.com/kb/en/mariadb/multi-source-replication/