RPL_SEMI_SYNC_MASTER_CLIENTS=1

Analysis: Uninstalling rpl_semi_sync_slave on slave
          will trigger removing the slave logic on Master which
          will reduce Rpl_semi_sync_master_clients by one number.
          But it happens asynchronously on Master. Having assert
          to check this value with zero will have problems on
          slow pb2 machines.

Fix: Change assert into wait_for_status_var condition.