Fn::GetAtt works incorrect when validating block_device_mapping

Step to reproduce the problem:
1. template as below:
heat_template_version: 2013-05-23
parameters:
  key_name: {type: string, default: heat_key}
  flavor: {type: string, default: m1.small}
  image_id: {type: string, default: c687fd63-97c5-40e6-86a4-cd2e81672f17}
  availability_zone: {type: string, default: nova}
resources:
  a:
    type: OS::Nova::Server
    properties:
      flavor: {Ref: flavor}
      key_name: {Ref: key_name}
      block_device_mapping:
        - device_name: vda
          volume_id: {'Fn::GetAtt': [disk, id]}
          delete_on_termination: false
      user_data:
        Fn::Base64: '80'
  disk:
    type: OS::Cinder::Volume
    properties:
      size: 2
      imageRef: {Ref: image_id}
      availability_zone: {Ref: availability_zone}
2. heat stack-create -f ....

Error found: Either volume_id or snapshot_id must be specified for device mapping vda

If we change:
          volume_id: {'Fn::GetAtt': [disk, id]}
to
          volume_id: {Ref: disk}
the error will disappear.

Final solution:

Disable volume_id: {'Fn::GetAtt': [disk, id]} in code and use volume_id: {Ref: disk} instead.