[Guestbook App] - Adding GRPC libs.

[smugcloud]

When running through the 'Getting Started' guide, the Guestbook app was not launching correctly because of the missing gRPC libraries:

Traceback (most recent call last):
  File "main.py", line 27, in <module>
    from vtdb import grpc_vtgate_client  # pylint: disable=unused-import
  File "/app/pkg/py-vtdb/vtdb/grpc_vtgate_client.py", line 24, in <module>
    import grpc
ImportError: No module named grpc

This PR modifies the Docker build process to include the gRPC libraries.