pdctl/command: add scheduler command

[huachaohuang]

Add command to show, add and remove schedulers.
Close #492

» help scheduler
show schedulers

Usage:
  pdctl scheduler
  scheduler [command]

Available Commands:
  add         add a scheduler
  remove      remove a scheduler

» help scheduler add
add a scheduler

Usage:
  add [command]

Available Commands:
  evict-leader-scheduler   add a scheduler to evict leader from a store
  grant-leader-scheduler   add a scheduler to grant leader to a store
  shuffle-leader-scheduler add a scheduler to shuffle leaders between stores
  shuffle-region-scheduler add a scheduler to shuffle regions between stores

» help scheduler remove
remove a scheduler

Usage:
  pdctl scheduler remove <scheduler>

[ngaut]

Well done!

[huachaohuang]

PTAL @siddontang @disksing

[disksing]

No need return.

[huachaohuang]

I think it's more consistent.

[disksing]

2017

[disksing]

I think path or uri would be a better variable name than prefix.

[disksing]

LGTM

[siddontang]

should read all r.Body, I remember if not, the connection may be leak even close the body.

[huachaohuang]

I can't find any resource about why I must read r.Body, any link?

[siddontang]

because we can't know whether the body has data or not.

In most of the cases, PUT returns no body, but it is still valid that PUT returns body.

[siddontang]

So the safest way here is to read body and discard it.

[siddontang]

Rest LGTM