Support multiple tpunodes / multiple runs of tpu_app

[concretevitamin]

sky tpunode  # ...Success.

sky tpunode -c new 

# The second command, however, reuses the tpu name "sky_tpu".
# which will give:

I 12-28 11:58:55 cloud_vm_ray_backend.py:560] Launching on GCP us-central1 (us-central1-a)
E 12-28 11:59:03 cloud_vm_ray_backend.py:511] Updated property [core/project].
E 12-28 11:59:03 cloud_vm_ray_backend.py:511] ERROR: (gcloud.compute.tpus.create) ALREADY_EXISTS: Resource 'projects/intercloud-320520/locations/us-central1-a/nodes/sky_tpu' already exists
E 12-28 11:59:03 cloud_vm_ray_backend.py:511] - '@type': type.googleapis.com/google.rpc.ResourceInfo
E 12-28 11:59:03 cloud_vm_ray_backend.py:511]   resourceName: projects/intercloud-320520/locations/us-central1-a/nodes/sky_tpu
E 12-28 11:59:03 cloud_vm_ray_backend.py:511]
I 12-28 11:59:03 cloud_vm_ray_backend.py:518] TPU sky_tpu already exists; skipped creation.

this means 2 host VMs are created but they connect (? to verify) to the same underlying TPU. called "sky_tpu".