[Serve] --purge is not respected for downing Serve controller

[romilbhardwaj]

Trying to delete the sky serve controller with sky down <controller> --purge from a changed identity results in a hard ClusterOwnerIdentityMismatchError and the cluster is not purged.

Probably need to update _hint_or_raise_for_down_sky_serve_controller to respect --purge.

This is for a serve controller running on kubernetes (on my local dev branch):

$ sky down sky-serve-controller-2ea485ea --purge
D 01-31 13:12:42 skypilot_config.py:139] Using config path: /Users/romilb/.sky/config.yaml
D 01-31 13:12:42 skypilot_config.py:143] Config loaded:
D 01-31 13:12:42 skypilot_config.py:143] {'serve': {'controller': {'resources': {'cloud': 'kubernetes'}}}}
D 01-31 13:12:42 skypilot_config.py:153] Config syntax check passed.
Traceback (most recent call last):
  File "/Users/romilb/tools/anaconda3/bin/sky", line 8, in <module>
    sys.exit(cli())
  File "/Users/romilb/tools/anaconda3/lib/python3.9/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/Users/romilb/tools/anaconda3/lib/python3.9/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/utils/common_utils.py", line 316, in _record
    return f(*args, **kwargs)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/cli.py", line 1222, in invoke
    return super().invoke(ctx)
  File "/Users/romilb/tools/anaconda3/lib/python3.9/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/romilb/tools/anaconda3/lib/python3.9/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/romilb/tools/anaconda3/lib/python3.9/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/utils/common_utils.py", line 337, in _record
    return f(*args, **kwargs)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/cli.py", line 2942, in down
    _down_or_stop_clusters(clusters,
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/cli.py", line 3128, in _down_or_stop_clusters
    hint_or_raise(controller_name)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/cli.py", line 3003, in _hint_or_raise_for_down_sky_serve_controller
    cluster_status, _ = backend_utils.refresh_cluster_status_handle(
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/utils/common_utils.py", line 337, in _record
    return f(*args, **kwargs)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/backends/backend_utils.py", line 2099, in refresh_cluster_status_handle
    record = refresh_cluster_record(
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/backends/backend_utils.py", line 2069, in refresh_cluster_record
    check_owner_identity(cluster_name)
  File "/Users/romilb/Romil/Berkeley/Research/sky-experiments/sky/backends/backend_utils.py", line 1571, in check_owner_identity
    raise exceptions.ClusterOwnerIdentityMismatchError(
sky.exceptions.ClusterOwnerIdentityMismatchError: 'sky-serve-controller-2ea485ea' (Kubernetes) is owned by account ['gke_skypilot-375900_us-central1-c_gkeusc2_gke_skypilot-375900_us-central1-c_gkeusc2_default'], but the activated account is ['gke_skypilot-375900_us-central1-c_gkeusc2_gke_skypilot-375900_us-central1-c_gkeusc2-skypilot-sa_default'].