BUGFIX: fixes a bug introduced by the use of get_ec2_creds in the rds module

Commit 12005a1 modified the rds module to use get_ec2_creds. Most of the ec2 modules use the ec2_access_key and ec2_secret_key nomenclature and get_ec2_creds extracts those module parameters. However, rds uses aws_access_key and aws_secret_key instead with an alias for ec2_*.

As a result, the rds module will not authenticate properly if aws_*_key are used, and will instead use only the AWS_* environment variables if they are defined.

This fix standardizes the AWS cloud module access and secret key naming convention on aws_access_key and aws_secret_key as more encompassing names.