Check for AWS keys using boto names

- see https://github.com/boto/boto#getting-started-with-boto

I haven't deep-dived into all the AWS code, but it seems like most (all?) ec2 modules use boto behind the scenes. If this is true, we might want to use the [boto.core.get_credentials()](https://github.com/boto/boto/blob/develop/boto/core/credentials.py#L147) method since they have a substantial search path for the keys.

I'm not sure how such a change would affect eucalyptus users though.