[Merge] ~xnox/livecd-rootfs:snap-preseed-channel-specification into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

Review: Approve

Looks OK to me, don't know if you want to get an architects view of the syntax or not...
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https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/385774
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