improve offset and size handling in disk config format

in looking at disk format information provided to curtin, i had some improvements i wanted:

http://paste.ubuntu.com/10939715/
^^ working from that.

offset:
 if 'offset' is present, it can be
   * human friendly unit (5M, 1G). these will be converted to bytes
   * integer : read as bytes
   * <name>+<number>
     name is a reference to an other entry 'id'
     here, number can be friendly (5M) or integer

  curtin quickly converts the provided input, converting 'offset' to bytes.

 if offset is not present, then the offset is assumed to be the end
 of the previous partition on that device in the list.

 if offset is not present for the *first* partition in the list, then it is assumed to be 1M. This allows users to basically specify sizes only and get sane behavior (while possibly wasting 1M).

  given:
    - {id: sda1, device: sda, offset: 1M, size: 10M}
    - {id: sda2, device: sda, size: +}
  result:
    - {id: sda1, device: sda, offset: 1M, size: 10M}
    - {id: sda2, device: sda, offset: 11M, size: <device-size - 11M>}

  given:
    - {id: x0, device: sda, size: 1M}
    - {id: x2, device: sda, size: 2M}
  result:
    - {id: x1, device: sda, offset: 1M, size: 1M}
    - {id: x2, device: sda, offset: 2M, size: 2M}

  A more complete and valid format is
    - {id: sda1, device: sda, offset: 1M, size: 10M, partition_id: 1}
    - {id: sda3, device: sda, offset: 511M, 1M, size: 10M, partition_id: 3}
    - {id: sda2, device: sda, offset: 11M, size: 400M, partition_id: 2}

partition_id:
  a.) ordered interpretation
      if 'partition_id' is not present it will be determined by order in the
      list for any partition on that device.
      given:
        - id: sda3
          device: sda
        - id: sda1 [gets partition_id=2]
      the result is;
        - {id: sda3, device: sda, partition_id: 1}
        - {id: sda1, device: sda, partition_id: 2}

      We do not do string based interpretation on ids. ie, the name 'sda2'
      does *not* imply the partition should be numbered 2.

  b.) if present it will be used, but must be present for all entries for a
      given device (this is just simplification to avoid having to check for
      a specified partition id that is not correct given ordered
      interpretatition)

      valid:
       - {id: sda1, device: sda, partition_id: 1}
       - {id: sda2, device: sda, partition_id: 2}

      valid and equal to above:
       - {id: sda1, device: sda}
       - {id: sda2, device: sda}

      error:
       - {id: sda1, device: sda, partition_id: 1}
       - {id: sda2, device: sda}