[Core] Support cross-cloud failover when labels are provided

[romilbhardwaj]

If a task specifies labels, it must currently specify the cloud to allow for checking labels since label validation varies across clouds. See discussion here.

We may need a way to allow cross-cloud failover when resources are specified.

Current workaround is to specify multiple clouds through any_of or ordered if failover with labels is desired:

resources:
  any_of:
    - cloud: gcp
      labels:
        cloud_name: gcp
    - cloud: kubernetes
      labels:
        my_domain/cloud_name: kubernetes