V2 instance creation requires explicit cluster information

[tseaver]

@garye The protos seem to indicate that an instance will get a default cluster created if nothing is specified, but:

raceback (most recent call last):
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/system_tests/bigtable.py", line 98, in setUpModule
    created_op = Config.INSTANCE.create()
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/bigtable/instance.py", line 362, in create
    request_pb, self._client.timeout_seconds)
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/.tox/system-tests/lib/python2.7/site-packages/grpc/beta/_client_adaptations.py", line 300, in __call__
    self._request_serializer, self._response_deserializer)
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/.tox/system-tests/lib/python2.7/site-packages/grpc/beta/_client_adaptations.py", line 198, in _blocking_unary_unary
    raise _abortion_error(rpc_error_call)
grpc.framework.interfaces.face.face.AbortionError: AbortionError(code=StatusCode.INVALID_ARGUMENT, details="Error in field 'clusters' : Need to have at least one cluster map element in CreateInstanceRequest.")

[garye]

Where were you looking in the protos? It mentions having to create one cluster here:
https://github.com/GoogleCloudPlatform/cloud-bigtable-client/blob/master/bigtable-client-core-parent/bigtable-protos/src/main/proto/google/bigtable/admin/v2/bigtable_instance_admin.proto#L104

[tseaver]

My mistake: I somehow imagined that cluster location and serve_nodes had server-side defaults.