[CLI] Add set,delete,current for manage context

[zimengyang]

Mange dispatch contexts by dispatch context

• Add set,delete and current for managing context

  $ dispatch context current
  $ dispatch context set <CONTEXT>
  $ dispatch context delete <CONTEXT>

• Listing all contexts remains as dispatch context

[neosab]

dispatch manage context set CONTEXT seems too long to me. Should we move context to a subcommand to dispatch? IMO, manage operates more on the dispatch installation but context is just a client configuration.

[zimengyang]

Yes, codes of manage context were removed. @berndtj