Add option to use a different KMS key when sharing

[fschroder]

This project gave me a great starting point, so foremost thank you for that!
Just as reported in the sister project (eg: awslabs/rds-snapshot-tool#60), my use case was to share a snapshot with a different account. Since some of the clusters are using the default KMS key this doesn't really work, as the destination account can never access the needed KMS key and therefore can't make a local copy.

To fix this I implemented an extra copy step in the source account (after the take snapshot) to bring all the snapshots to use the same KMS key and then share them. It's potentially less efficient as it generates an extra snapshot copy, but it makes the process generic after that point.

Maybe it's something that can be added to the project, but it would make the generic solution more complex.

[sbranchaw]

I created a PR to add this functionality. We're using it in production at my company. Feedback welcome!

And like fschroder, I appreciate this project, which got my company about 80% of the way to where we needed to be with regards to a snapshot-copying solution.