Refactor with minor improvements, and add README

[stevapple]

This PR intends to re-organize all the codes into different "submodules". Most notably, it removes all global variables and wraps all the functionalities into classes with minor changes to the behavior, enabling more precise testing. recovery-token is added as an optional configuration, and logger is no longer required.

This PR also adds a concrete README on all the configurable options and the API design.

[stevapple]

There should be subsequent "breaking" PRs, including a better config representation (maybe TOML?), transferring some responsibilities to the API server, more customization, etc.

[stevapple]

cc @taoky . I’m especially unsure about the role of token in the API server and Vlab recovery system.

[taoky]

cc @taoky . I’m especially unsure about the role of token in the API server and Vlab recovery system.

API server and recovery backend are two different applications, though currently they're using the same token...

[stevapple]

API server and recovery backend are two different applications, though currently they're using the same token...

So the API server also requires token for safety reason?

[iBug]

So the API server also requires token for safety reason?

I'm fine if you're splitting it into two tokens and mark RecoveryToken as optional.

[taoky]

API server and recovery backend are two different applications, though currently they're using the same token...

So the API server also requires token for safety reason?

Yes.

[iBug]

I'm especially unsure about the role of token in the API server and Vlab recovery system.

The recovery-sshd server lives in another private repo. It accepts plain SSH requests with username being one of recovery, console or serial matching those described here, and password being <VMID> <token> (concatenation of both items separated by a single space). The token is there against arbitrary access as these recovery access methods are inherently of very high privileges in the VM.

Not sure why we didn't open-source recovery-sshd. The repo contains no secret beyond what could certainly be deemed "implementation details".

[stevapple]

I'm fine if you're splitting it into two tokens and mark RecoveryToken as optional.

Done.

[iBug]

@taoky We can merge this PR at any time, but before we deploy it you'll have to update our Django server to adapt to the new API request/response schema.

[stevapple]

@iBug I don’t think we introduced any breaking change since v0.0.7…? All the features and improvements are purely additional.

[iBug]

Would you like a chance to reorganize these commits so that they're suitable for a merge commit? Or would you mind if this large change shows as a single commit in the history?

[stevapple]

I would always prefer a squash merge:) If you don’t mind, please clean up the message body so it won’t be flooded by those commits.