Include external IP address of AWS analysis instance when applicable

It would be helpful if after finding, or creating the analysis instance via [`GetOrCreateAnalysisVm()`](https://github.com/google/cloud-forensics-utils/blob/master/libcloudforensics/providers/aws/internal/account.py#L459) that the returned data included the external IP address of said instance (e.g. EIP).

This metadata may not always be present, but when it's there then it would be useful to present that from the [`AWSInstance`](https://github.com/google/cloud-forensics-utils/blob/master/libcloudforensics/providers/aws/internal/ec2.py#L26) class so a user may more easily pivot to a remote connection, if warranted.