Allow AMI block device mapping to be created without snapshot

[ghost]

This issue was originally opened by @otterley as hashicorp/packer#5290. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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I have a use case where I have two launch_block_device_mappings, /dev/sda1 (root device) and /dev/sdb (data device). The data device exists during the build phase to satisfy some build-time prerequisites, but the actual volume content can be discarded at the the end of the build. The device can also be quite small.

Meanwhile, there are two ami_block_device_mappings of the same device names, /dev/sda1 and /dev/sdb. I'd like /dev/sdb NOT to be associated with a snapshot, since at launch time the instance should have a fresh volume. But Packer seems to always associate it with a snapshot because the device was mapped at launch time. Setting snapshot_id to "" seems to have no effect.