Default block_device_mapping fields

[tyler-ball]

Currently if you want to overwrite the block_device_mappings, you are required to specify all 3 fields:

block_device_mappings:
  - ebs_device_name: /dev/sda1
    ebs_volume_size: 8
    ebs_delete_on_termination: true

We should default ebs_volume_size and ebs_delete_on_termination if they are not provided, so the following config would work:

block_device_mappings:
  - ebs_device_name: /dev/sda1
    # ebs_volume_size will be defaulted to 8
    # ebs_delete_on_termination will be defaulted to true