Adding aws_access_key and aws_secret_key aliases to ec2_group arguments

There is inconsistency in the documents about using aws_ or ec2_ prefix for secret_key and access_key.  Realize this is water under the bridge, but think the safest course is to alias all of the ec2 modules both ways.  I had a case where I was assuming aws_*_key variables were being used when it was in fact pulling EC2_*_KEY from elsewhere.  

We may want to change the documentation to all read one way even if it is the preferred alias.  Probably ec2_*_key since not all EC2 implementations are Amazon.