[Core] Allow boolean-like strs in resources.labels

[romilbhardwaj]

A yaml like this would fail:

resources:
  cloud: kubernetes
  labels:
    my-label: 'True'

Because when the cluster YAML was dumped for the provisioner in ~/.sky/generated/, it would remove the quotes and write my-label: True. This would be parsed as a bool value (instead of str) by yaml.safe_load, which would cause issues with downstream code (e.g., Kubernetes python API does not allow submitting bools as label values).

This PR adds single quotes around all dumped labels to make sure they are parsed as strings by the provisioner.

Tested:

• YAML above on Kubernetes.

[romilbhardwaj]

Thanks @Michaelvll! Using jinja logic and tested again.

[Michaelvll]

Thanks for the update @romilbhardwaj! LGTM.