livecd-rootfs must stop using manual seeds, and always use `snap prepare-image --classic`

[Dimitri John Ledkov]

$ snap prepare-image --classic /var/lib/snapd/assertions/asserts-v0/model/16/generic/generic-classic/active `pwd` --snap core18=latest/stable --snap subiquity=latest/stable/ubuntu-20.04 --arch s390x

is really cool and creates lovely things like:

snaps:
- name: core18
  snap-id: CSO04Jhav2yK0uz97cr0ipQRyqg0qQL6
  channel: latest/stable
  file: core18_1704.snap
- name: subiquity
  snap-id: ba2aj8guta0zSRlT3QM5aJNAUXPlBtf9
  channel: latest/stable/ubuntu-20.04
  classic: true
  contact: https://bugs.launchpad.net/subiquity
  file: subiquity_1704.snap

It has:
- incremental seeding
- validating seeds

However it lacks:
- cohort support
- auto-seeding snapd

But it does extensive validation and prevents broken seeds.

[Dimitri John Ledkov (xnox)]

Related https://forum.snapcraft.io/t/seeding-snaps-that-plug-the-content-interface/5579/14

[Robert C Jennings (rcj)]

This is great and I'd love to see us move to this from what we are doing. Parallel builds for the ubuntu-cpc project have an absolute requirement for cohort support; so that requirement will need to be driven with snapd.