Regenerate Bigtable V2 protos

[tseaver]

(after the version landed in #1903).

@sduskis reports:

There were some relatively minor changes in the v2 protos. Mostly the changes were in the comments. There were minor changes to rename fields from "name" to "parent" to better reflect the meaning of some of the proto elements. The latest version of the protos can be found in the official
google api repository at https://github.com/googleapis/googleapis/tree/master/google/bigtable.

[tseaver]

My setup for protoc v3 and the grpc_python_plugin is broken: the gRPC portion fails to generate with a "Missing input file" error.

I scraped the needed gRPC stub code from #1895, but we really need to get #1384 or #1482 nailed down.

[tseaver]

Looks like grpcio-tools >= 0.14.0 can carry our water:

$ python2.7 -m virtualenv /tmp/grpc_python
$ /tmp/grpc_python/bin/pip install grcpio grpcio-tools
$ git clone https://github.com/google/googleapis/
$ mkdir generated
$ /tmp/grpc_python/bin/python -m grpc.tools.protoc \
  --proto_path=googleapis --python_out=xxx --grpc_python_out=xxx \
  googleapis/google/bigtable/v2/bigtable.proto

[tseaver]

Via #1925.

[tseaver]

gRPC generation for $(GOOGLEAPIS)/google/longrunning/operations.proto is brokey:

  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/bigtable/client.py", line 344, in start
    self._operations_stub_internal = self._make_operations_stub()
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/bigtable/client.py", line 307, in _make_operations_stub
    OPERATIONS_API_HOST_V2, OPERATIONS_API_PORT_V2)
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/bigtable/client.py", line 467, in _make_stub
    return stub_factory(channel)
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/bigtable/_generated_v2/operations_grpc_pb2.py", line 238, in beta_create_Operations_stub
    ('google.longrunning.Operations', 'CancelOperation'): CancelOperationRequest.SerializeToString,
NameError: global name 'CancelOperationRequest' is not defined

Adding the imports by hand to push forward on system tests:

--- a/gcloud/bigtable/_generated_v2/operations_grpc_pb2.py
+++ b/gcloud/bigtable/_generated_v2/operations_grpc_pb2.py
@@ -1,4 +1,12 @@
-
+from google.longrunning.operations_pb2 import (
+    CancelOperationRequest,
+    DeleteOperationRequest,
+    GetOperationRequest,
+    ListOperationsRequest,
+    ListOperationsResponse,
+    Operation,
+    google_dot_protobuf_dot_empty__pb2,
+)
 from grpc.beta import implementations as beta_implementations
 from grpc.beta import interfaces as beta_interfaces
 from grpc.framework.common import cardinality