[master] KUDU-3389 support turning on/off auto rebalancer at runtime

kudu-master was able to run the auto-rebalancer, but toggling it on/off
in runtime hadn't been supported. This patch use the existing
--auto_rebalancing_enabled flag and support be set it in runtime via the
SetFlag() RPC, toggling the auto-rebalancer on/off dynamically.

Change-Id: I0ecee5d8b0b2344c3ad190548526ed6a2551cfdb
Reviewed-on: http://gerrit.cloudera.org:8080/18923
Reviewed-by: Yuqi Du <shenxingwuying@gmail.com>
Tested-by: Kudu Jenkins
Reviewed-by: Yingchun Lai <acelyc1112009@gmail.com>
Reviewed-by: Mahesh Reddy <mreddy@cloudera.com>
Reviewed-by: Alexey Serbin <alexey@apache.org>
3 files changed