Add aws:// target

[clintoncwolfe]

🚧 DO NOT MERGE until chef-boneyard/inspec-aws-old#149 is resolved 🚧

The inspec-aws resource pack currently works "by accident"; all resources ignore the inspec runner (and anything it knows about the transport), and simply read credentials from the AWS_* environment variables. Inspec thinks it is running in local mode the whole time.

We've discussed internally having an explicit aws:// target, with possibilities of using various parts of the URI for various credentials.

This issue may need to be split, for an RFC to determine the URI format, then an implementation issue.

Refs inspec/inspec#2233