provide min_disk attribute for images

[Peter Sabaini]

Currently images are created with a min_disk=0 in glance:

$ glance image-show e033e913-73f2-4357-bc2a-f7afa8224689
+------------------------------+-----------------------------------------------------+
| Property | Value |
+------------------------------+-----------------------------------------------------+
...
| min_disk | 0 |

It would be good to have this set to the actual disk size where available. Eg. by extracting it via qemu-img

[Scott Moser (smoser)]

2 ways we could do this:
a.) simplstreams data should have it (in the source data on cloud-images.ubuntu.com)
    and then glance mirror would just trust that
b.) if qemu-img is availalbe we could use it.

note, that qemu-img now has '--output-format=json'.