[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:no-snaps-ubuntu-cpc-minimized into livecd-rootfs:ubuntu/master

[Tobias Koch]

Review: Needs Fixing

IMO this needs the following changes

1. Instead of removing 'config/seeded-snaps' in 'auto/config', it should be truncated, so that 'snap_prepare' keeps being called.

2. Installation of the core snap via '_snap_preseed $CHROOT_ROOT core stable' needs to be removed from 'snap_prepare' and

3. instead, _snap_preseed should be modified to use 'snap-tool info <snap>' to determine which core is needed and seed that as needed.

That way a) hooks do not have run call snap_prepare, if they do not need to change the model assertion, b) the correct core will be seeded including core18 if necessary c) core will show up in seed.yaml and thus also in the manifest.
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/364222
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