Current rebalance throttle options: lazy/normal/aggressive may not always be
sufficient for the purpose of throttling.  In our recent test, we observed for
certain setups, normal and aggressive modes behaved similarly consuming full
disk bandwidth. So in cases like this admin should be able to  tune it
down(or vice versa) depending on the need.

Along with old throttle configurations, thread counts are tuned based on number.
e.g. gluster v set vol-name cluster-rebal.throttle  5.

Admin can tune up/down between 0 and the number of cores available.

Note: For heterogenous servers, validation will fail on the old server if "number"
is given for throttle configuration.
The message looks something like this:
"volume set: failed: Staging failed on vm2. Error: cluster.rebal-throttle should be {lazy|normal|aggressive}"

Test: Manual test by logging active thread number after reconfiguring throttle option.
testcase: tests/basic/distribute/throttle-rebal.t

Change-Id: I46e3cde546900307831028b344ecf601fd9b02c3
BUG: 1438370
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: https://review.gluster.org/16980
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
Smoke: Gluster Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>

Throttle value will be "normal" by default. For throttling down,
a thread will be put in to sleep. And for throttling up,
gf_defrag_process_dir will wake up the sleeping threads.

Change-Id: I74d530e3effd6e60e6eec81ccc8ff65789fa9c13
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: http://review.gluster.org/10526
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Raghavendra G <rgowdapp@redhat.com>