Allow Amazon-Import to only import to S3

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#### Description

Requesting to be able to have 'Amazon Import' post-processor to upload a created image to S3, skipping the AMI conversion step.
Many people are starting to use S3 as a 'static' content library for vSphere. Being able to have packer create images and dump them directly to an S3 bucket would be great.

#### Use Case(s)

Any customer that often builds and destroys vSphere environments are starting to use S3 as a static content library for vCenter.

#### Potential configuration

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#### Potential References