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

Commit https://github.com/ansible/ansible/commit/12005a1cd0e234f64ac2d7c865a7cbd77ea4fbc4 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 change modifies get\_ec2\_creds to look for the common aliases for ec2\_access\_key, ec2\_secret\_key, and region, which both fixes the rds authenticate and further standardizes the ec2 functionality.

A more invasive fix would change *all* of the ec2 and other aws modules to use the more encompassing aws\_*\_key nomenclature but I am concerned that such a change would be difficult to test thoroughly enough to ensure backwards compatibility.