New rebalance setting

New rebalance setting, "Max vbucket moves per node during rebalance", takes integer values between 1-64. 

This setting may help make rebalance faster/slower by increasing/decreasing the number of vbucket moves that can happen involving a node. Increasing vbucket moves may(highly likely) come at a cost of increasing the resource(cpu/memory/disk etc) pressure on a node.

Note: increasing the  "Max vbucket moves per node during rebalance" doesn't guarantee faster rebalance , it is still dependent on other factors, such as nodes involved in the vbucket moves, type of rebalance, presence of views etc.

Also, the setting should also come with a warning that,

"Changing this setting can leads to degradation of front-end ops, due to changes caused in resource utilization on the node(s)."

Can get/set this setting as,

$ curl -X GET -u Administrator:asdasd http://localhost:9000/settings/rebalance {"rebalanceMovesPerNode":4} 

$ curl -X POST -u Administrator:asdasd -d 'rebalanceMovesPerNode=64' http://localhost:9000/settings/rebalance 

{"rebalanceMovesPerNode":64} 

$ curl -X GET -u Administrator:asdasd http://localhost:9000/settings/rebalance {"rebalanceMovesPerNode":64} 

$ curl -X POST -u Administrator:asdasd -d 'rebalanceMovesPerNode=65' http://localhost:9000/settings/rebalance 

{"errors":{"rebalanceMovesPerNode":"The value must be in range from 1 to 64"}} 

Similar, can be done through the UI.