Support registry resource

[Yancey1989]

• Add registry: paddlecloud registry --name test --username test --password test --server x.y.z
• Delete registry: paddlecloud delete registry ${registry-name}
• List registry: paddlecloud get registry

Fixed #140

[typhoonzero]

Please also add docs in doc/usage_cn.md

[typhoonzero]

What does the second argument "PFS" mean?

[Yancey1989]

The second argument of subcommands.Register is group, if we add a subcommand with a specify a group name, these subcommands will be explained with a group name before, such as:

Subcommands:
	commands         list all command names
	delete           Delete the specify resource.
	file             Simple file operations.
...

Subcommands for PFS:
	cp               uoload or download files
	ls               List files on PaddlePaddle Cloud
...

[typhoonzero]

I see.

[typhoonzero]

LGTM. Still, commands like paddlecloud registry ... means add, but there's no verb in this command