Finished implementation of RDS's DescribeDBLogFiles

This commit implements the missing arguments for get_all_logs.

It introduces two slight API changes. They're the right thing to do, but I can revert those parts if needed. 

a) dbinstance_id is no longer optional. It's a required field in the docs, so there's no reason for it to be optional.

b) max_records was previously hardcoded to 26. It's actually an optional field, so I've changed to be so.