[Merge] ~xnox/livecd-rootfs:snap-prepare-image into livecd-rootfs:ubuntu/master

[Dimitri John Ledkov]

The proposal to merge ~xnox/livecd-rootfs:snap-prepare-image into livecd-rootfs:ubuntu/master has been updated.

Commit message changed to:

* Add alternative way to seed snaps, using `snap prepare-image` which seeds all snaps in one go and also performs the snap-preseed for accelerated first boot.

* Wipe existing seeds on ubuntu-server-live image, to be the first image to try this, before switching other images to use this.

* This will allow specifying "* snap:name=track/risk/branch" in the seeds, to correctly seed things of different tracks if need be, on per-snap basis.

For more details, see:
https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/383947
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Your team Ubuntu Core Development Team is requested to review the proposed merge of ~xnox/livecd-rootfs:snap-prepare-image into livecd-rootfs:ubuntu/master.