ebs-surrogate: Make AMI registration optional

_This issue was originally opened by @cetex as hashicorp/packer#4899. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

I want to build images for netboot (pxe) for our on-premise hardware using packer and i want to do this on Amazon EC2. Since the result of the build won't run in AWS it would be great if taking a snapshot and registering it as an AMI was optional for the amazon builders.

If this was optional i could use something like Amazon ebs surrogate builder:

- Run debootstrap to create a chroot on the mounted ebs volume.
- Run my ansible playbook inside the chrooted directory.
- Run a script to pack the entire directory up into a cpio/gz archive.
- Send the created output files to s3 or something similar.
- **Skip snapshot + AMI registration <- this is what's missing.**
- Cleanup / delete instance and volume directly as the resulting images would be useless for any purpose on AWS.