[Cloud Build] Add tags to each build configuration

[markmandel]

There's a tags section in:
https://cloud.google.com/build/docs/build-config-file-schema

I'm thinking our builds should have some sort of tag that identifies it as global-game but also which build it is.

Maybe:

tags:  ['global-game-demo', 'services'] 

For the services, build, and then platform for the platform build, etc.

[markmandel]

Got this going locally. Attempting to combine this with #104