Support custom Systems Manager Document

[Chris Long]

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Description

Can you please support the ability to use a specified Systems Manager document when provisioning with ssm on AWS. Currently, the functionality is hard coded to the AWS managed document named AWS-StartPortForwardingSession

This initial enhancement would be great if it just handled a custom document but the parameters for the document would be identical to the AWS managed document. I can see further enhancements to supporting custom documents in the future which can be handled by #417.

Use Case(s)

Some companies strictly control which documents are allowed and when updates should be adopted. An AWS owned document does not let us control those aspects.

Potential configuration

source "amazon-ebs" "image" {
  ...
  communicator = "ssh"

  ssh_interface        = "session_manager"
  ssh_username         = "someuser"
  iam_instance_profile = "SomeInstanceProfileName"
  
  ssm_document = "MyPortForwardingDocument"   # Name of custom document to use
}

Potential References

https://docs.aws.amazon.com/systems-manager/latest/userguide/documents.html