Pass empty string to clear mantle balancer
Currently we can use "ceph fs set cephfs balancer <lua_conf_file>" to set mantle balancer. Then MDS balancer will try to use this mantle balancer firstly if MDS finds its balancer is not empty when handling hearbeat. If we want to switch back to old balancer, we need to set balancer to some strings which means an non-existed lua file to let MDS failing to reload it. But MDS still has to try reloading this non-existed lua file from OSD every time.
So we add a "clear_balancer" sub-command to clear mantle balancer, then MDS will see its balancer is empty again and won't send read OP to OSD every time.