[aws|rds] implement describe_db_log_files

This change provides access to the describe_db_log_files request for AWS RDS, api version 2013-05-15.

I have attempted to include tests but I am not certain of the actions that would be taken against our AWS account so I have not run the suite. I wrote my own small script (not committed, of course) to verify that the implementation is working.

I'd appreciate feedback on the tests (and the rest of it, naturally).