gadget.yaml should specify disk/volume image sizing behavior

[Barry Warsaw]

Currently, ubuntu-image calculates how big a volume/disk because there's no way to specify the intended size in the gadget.yaml. For images booted on physical devices, cloud images, and MAAS, the resulting disk.img is automatically resized on first boot. This is a good thing because it allows us to create smaller images without nul padding.

But this is problematic for other images such as kvm disks because the automatic resizing won't happen there. In cases like this, we need to be able to specify the final size of the disk image exactly.

In the short term, LP: #1632085 was added to provide an optional `--image-size` cli argument to ubuntu-image which would override the calculated size and generate a disk image of exactly the given size, as long as that was greater than the calculated size (a smaller --image-size is ignored).

Both the intended disk image size and the first boot resizing behavior should be specifiable in the gadget.yaml so authors can explicitly describe how their images should work. This bug tracks this RFE.

Long thread which starts here:

https://lists.snapcraft.io/archives/devices/2016-October/000087.html

Relevant messages in the thread:

https://lists.snapcraft.io/archives/devices/2016-October/000128.html
https://lists.snapcraft.io/archives/devices/2016-October/000130.html
https://lists.snapcraft.io/archives/devices/2016-October/000117.html

[Zygmunt Krynicki (zyga)]

Assigning to Maciek for evaluation. This bug is fairly old and perhaps is no longer relevant.