Pin grpcio and grpcio-tools versions

[martinvol]

Pinned the version to the latest one to avoid install irreproducibility and save users with grpcio-tools==1.27.0 and grpcio==1.27.0 installed.

See grpc/grpc#21922

Fixes #10377 🦕

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[busunkim96]

@crwilcox @tseaver Does this file need to be moved to the bigtable repo?

[tseaver]

Actually, the Makefile.bigtable_v2 file is a fossil, and should just go away: we haven't used it for generating anything in years.

@martinvol I'm not sure how you've been triggering the missing-grpc-version problem, but it cannot have been here: that file was always an internal-development-only tool, and never had anything to do with how google-cloud-bigquery got installed in your environment.