Simplify the parameters

## Feature Request
Simplify the parameters.

### Describe your feature request related problem
After added a new tikv node, I tried to rebalance the regions (moving some regions to this new one).
I google & read a bunch of docs and tuned a few parameters and still can't make it work.

I'm thinking maybe we should simplify the configuration for the new user like me.

For example, for rebalancing regions job, just provide a parameter like `tikv_rebalance_region_speed` and the valid value is 1 to 10
If I want to evenly distribute all regions to new tikv ASAP. I can set it to 10. Otherwise I can set it to 1.

Then please auto adjust all related parameters like `min-balance-diff-ratio`, `leader-schedule-limit`, etc. and do the math (https://github.com/pingcap/tidb/issues/2581) in the background.

Only the advance user should care those complicate calculation.

Oracle database did this very well. Only exposes the easy parameters to user, and hides the hard parameters.

No files, tests, or entry points are identified. Start by reviewing the existing region-rebalancing parameters, including min-balance-diff-ratio and leader-schedule-limit, and the calculations discussed in issue 2581. Done would require an agreed design for a simplified speed parameter and its relationship to the advanced settings.

Written by the indexing model from the issue text.