[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:lp-1844498 into livecd-rootfs:ubuntu/master

[Julian Andres Klode]

More extensive review:

- Matches the existing addition of groups in cpc
- New script for server does the same thing, has the right permission, so looks ok
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/373789
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