Allow multiple storage device mappings

[lyubomirdimov]

In certain cases, the launch template should create multiple device mappings. Such case is Bottlerocket Custom AMI, which requires mapping for /dev/xvda and /dev/xvdb (https://github.com/bottlerocket-os/bottlerocket#default-volumes).

The code at the moment, provides a static mapping for /dev/xvda:

  block_device_mappings {
    device_name = "/dev/xvda"

    ebs {
      volume_size = var.disk_size
      volume_type = var.disk_type
      kms_key_id  = var.launch_template_disk_encryption_enabled && length(var.launch_template_disk_encryption_kms_key_id) > 0 ? var.launch_template_disk_encryption_kms_key_id : null
      encrypted   = var.launch_template_disk_encryption_enabled
    }
  }

It would be great if we can make those mappings dynamic. I will submit a PR soon with the requested changes.

Update
PR - #75

Additional Context

Related issue, which I recently closed - #72