Add root_device_name to amazon-import

I did not find this feature discussed for amazon-import, but I do see it for other parts of the amazon plugin (the chroot builder, for instance). It _may_ be possible to do this with an existing feature, but I have not been able to figure that out.

#### Description

Please add the equivalent of `root_device_name` to the amazon-import post-processor.

#### Use Case(s)

This configuration item is necessary for multi-partition images that are built with other packer builders where the root partition is not `/dev/sda1`.

#### Potential configuration

```
,{
"type": "amazon-import",
"profile": "serviceAccount",
"region": "us-east-1",
"s3_bucket_name": "{{user `s3bucket`}}",
"s3_key_name": "{{user `template`}}-disk001{{user `suffix`}}.vmdk",
"license_type": "BYOL",
"tags": {
"Name": "{{user `template`}}",
"Description": "packer amazon-import {{user `template`}} @ {{timestamp}}"
},
"format": "vmdk",
"architecture": "x86_64",
"boot_mode": "uefi",
"root_device_name": "/dev/sda2",
"ami_name": "{{user `template`}}"
}
```