Support to manage virtual cluster in runtime.

[squirrelsc]

Virtual cluster is used to allocate resource. But it can be updated in config only. It's not easy to use. So it's better to be able to add/delete/update virtual cluster and update capacity in runtime.

[mzmssg]

The bot of virtual cluster is Yarn scheduler queue, which support update actually. This feature in PAI needs some workload and may be added in future.

[mzmssg]

Finished in #1985