[Merge] ~mvo/livecd-rootfs:snap-validate-seed into livecd-rootfs:ubuntu/master

[Robert C Jennings]

Review: Needs Fixing

This will not catch the issue in bug #1828500, for that you need to add this check as the last line of the "snap_preseed" function in live-build/functions.  Snaps are added by "preinstall_snaps" in live-build/auto/build but they are also added by binary hooks in separate chroots (which this MP will currently miss).
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https://code.launchpad.net/~mvo/livecd-rootfs/+git/livecd-rootfs/+merge/370065
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