Bigtable: 'test_bigtable_list_tables' snippet flakes with '504 Deadline Exceeded'.

[tseaver]

From this Kokoro failure:

__________________________ test_bigtable_list_tables ___________________________
    def test_bigtable_list_tables():
        from google.cloud.bigtable import Client
        from google.cloud.bigtable import column_family
        client = Client(admin=True)
        instance = client.instance(INSTANCE_ID)
        table = instance.table("to_list")
        max_versions_rule = column_family.MaxVersionsGCRule(2)
>       table.create(column_families={"cf1": max_versions_rule})
docs/snippets.py:358:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
google/cloud/bigtable/table.py:251: in create
    initial_splits=splits,
google/cloud/bigtable_admin_v2/gapic/bigtable_table_admin_client.py:340: in create_table
    request, retry=retry, timeout=timeout, metadata=metadata
../api_core/google/api_core/gapic_v1/method.py:143: in __call__
    return wrapped_func(*args, **kwargs)
../api_core/google/api_core/retry.py:273: in retry_wrapped_func
    on_error=on_error,
../api_core/google/api_core/retry.py:182: in retry_target
    return target()
../api_core/google/api_core/timeout.py:214: in func_with_timeout
    return func(*args, **kwargs)
../api_core/google/api_core/grpc_helpers.py:59: in error_remapped_callable
    six.raise_from(exceptions.from_grpc_error(exc), exc)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
value = DeadlineExceeded(u'Deadline Exceeded',)
from_value = <_Rendezvous of RPC that terminated with:
	status = StatusCode.DEADLINE_EXCEED...all.cc","file_line":1046,"grpc_message":"Deadline Exceeded","grpc_status":4}"
>
    def raise_from(value, from_value):
>       raise value
E       DeadlineExceeded: 504 Deadline Exceeded
.nox/snippets-2-7/lib/python2.7/site-packages/six.py:737: DeadlineExceeded