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

[Robert C Jennings]

Binary hooks that add pre-seeded snaps also need to add a call to 'snap debug validate-seed' and we risk missing this addition to new and existing ones.  This was the reason I requested the check in snap_preseed() rather than in preinstall_snaps().  It would cause the validation to run as each snap is added, but I would prefer, that if it works well enough, to missing call sites when someone adds a snap_preseed() and doesn't know to also add a new 'snap debug validate-seed' call.
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https://code.launchpad.net/~mvo/livecd-rootfs/+git/livecd-rootfs/+merge/370065
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