Implement config validation

[iojw]

Closes #2474

Implements validation for the ~/.sky/config.yaml file. If there is an error, an exception is thrown and the user is warned.

To avoid circular imports, I had to refactor the code a bit, moving validation code into common_utils and moving the imports in sky.util.schemas to within the functions.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Pass existing config tests, added new unit tests
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Thanks for submitting the PR @iojw! The code looks mostly good to me, but there seems to be a circular import, when I tried sky check with the current PR.

sky/data/storage.py", line 49, in <module>
    str(clouds.AWS()),
AttributeError: partially initialized module 'sky.clouds' has no attribute 'AWS' (most likely due to a circular import)

[iojw]

Updated to throw an exception and also to fix the circular import issue mentioned (it's caused by the kubernetes_utils import in schemas.py so I've moved the enums to a separate module).

Strangely, I'm unable to reproduce it locally, so could you help to verify that it is fixed @Michaelvll ?

[Michaelvll]

Thanks for identifying the issues and fixing them @iojw! It looks mostly good. Left some minor comments. cc'ing @concretevitamin to see if he has some additional test to be run. : )

[Michaelvll]

Thanks for adding the schema check @iojw! LGTM with an additional comment. Would be nice to do some more manual test to avoid any accident false positive schema alert.

[romilbhardwaj]

Thanks @iojw! Left some comments on the k8s config docs, otherwise LGTM!

[iojw]

Thank you @romilbhardwaj for taking a look! I'll merge this in soon if there are no further comments.