Allow to configure scale down amount (count or percentage)

[ichekrygin]

For larger clusters (100+ nodes), scaling down (say to 20 nodes) - takes an order of several hours due to a conservative nature of scale-down step in autoscaler. It would be very helpful to be able to scale down by more than 1 instance a the time, by providing a step-down configuration value expresses either in:

• count: absolute number of instances to scale down by (for example by 2, 5, etc)
• percentage: relative number of instance based on total (for example by 5%, 10%, etc)