Add cli flag to discover AWS credentials via AWS SDK default credential chain

[eldondevsit]

Abstract

The AWS SDK has a method of resolving credentials based on a default credentials chain. It would be nice to pass a single flag instead of having to pass flags even if the standard environment variables are set, or to use the IMDS directly.

Proposal

Add a flag which instructs terracognita to use the aws golang sdk to
discover credentials via the default credential provider chain, including things like IMDS, etc.

[eldondevsit]

I can try to get together a PR for this if that would be of interest.