Make S3 bucket creation optional

[mvandenburgh]

Currently the module allows the user to disable provisioning of Heroku Postgres, CloudAMQP, and Papertrail resources (by setting these variables to null), but doesn't expose the same option for S3. Granted an empty S3 bucket doesn't incur any costs like those other services would, but it would be nice to avoid the clutter/possible confusion in the AWS console on projects that don't use S3, or those that manage their own S3 buckets (like DANDI).