Update cli.py - gcloud compatibility

[7flash]

This is very useful tool, and since I want it to store my images in s3-compatible google storage bucket, I added this parameter to provide "https://storage.googleapis.com" as an endpoint url. Probably, should also update dogsheep-photos s3-auth command.

Also, as a side note, in order to find s3-compatible credentials from your gcloud, follow to this page: https://console.cloud.google.com/storage/settings;tab=interoperability