[Config] Add multiple clusters

[michaelzhiluo]

SkyShift config in ~/.skyconf/config.yaml should have the option to add multiple clusters in its config file.

[AlexCuadron]

This will be solved in #171 tho

[michaelzhiluo]

Lets discuss design choice about this, whether it should only include other Skyflows or also other cluster managers...

I'm leaning no for other cluster managers after more thought.