[Merge] ~tobijk/livecd-rootfs/+git/livecd-rootfs:feature/snap-tool-prefer-stable-for-core into livecd-rootfs:ubuntu/master

[Łukasz Zemczak]

Review: Approve

Ok, this seems like a nice workaround for the problem, +1. I'm still investigating actually using the new snap prepare-image --classic for seeding, but from what I see it wouldn't solve our current problem anyway (it's less smart than we'd like it to be). 
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https://code.launchpad.net/~tobijk/livecd-rootfs/+git/livecd-rootfs/+merge/364800
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