support turning on/off auto rebalancer at runtime

kudu-master supports auto-rebalancer, but it must be decided whether enable it before kudu master starting.

If users want to change the flag, they should change the gflag and restart masters.

So supporting to turn on/off `auto rebalancer` when kudu-master running is a good idea.

We can use gflag --auto_rebalancing_enabled to do this job.

https://gerrit.cloudera.org/c/18923/

The patch: https://gerrit.cloudera.org/c/18679/