Add a gangue subcommand to clean up files in GCS

[margamanterola]

Add a gangue subcommand to clean up files in GCS

By adding developer builds, we've started generating a lot of files in GCS that are actually ephemeral. This change adds a new prune subcommand to the gangue command (until now only get existed). This subcommand can be used to delete files under a specific path if they are older that some amount of days.

How to use / Testing done

go build -o bin/gangue github.com/coreos/mantle/cmd/gangue
bin/gangue prune flatcar-jenkins developer/sdk \
    --json-key /path/to/creds/gce-service-account.json --days=15

This will delete all files older than 15 days under the developer/sdk path. I've run this for sdk and for boards and verified that the files deleted were correct.

This change is part of the clean up needed for building PR branches (kinvolk/PROJECT-flatcar-linux#294)

[sayanchowdhury]

LGTM

Edit: Missed to check the approve button (should be approve instead of comment)