[Merge] ~jchittum/livecd-rootfs:mount-faux-cgroup-for-snapd into livecd-rootfs:ubuntu/master

[John Chittum]

John Chittum has proposed merging ~jchittum/livecd-rootfs:mount-faux-cgroup-for-snapd into livecd-rootfs:ubuntu/master.

Commit message:
Mount cgroup2 type for snapd
    
LP: 1944004 described an issue where a libc transition caused snapd
seccomp profiles to reference a path that no longer existed, leading to
permission denied errors. The committed fix for snapd then raised an
issue where running `snapd debug seeding` would present a
preseed-system-key and seed-restart-system-key due to a mismatch
between the running kernel capabilities and the profiles being loaded by
snapd. By mounting a cgroup2 type to /sys/fs/cgroup, the capabilities
match for snapd as mounted in the chroot. This is done similarly to
live-build/functions:138-140 where apparmour and seccomp actions are
mounted after updating the buildd.

Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~jchittum/livecd-rootfs/+git/livecd-rootfs/+merge/409359

worked with anonymouse67, stolowski, and pedronis to identify the change. The change needed in livecd-rootfs presented itself during CPC testing, and my testing here is to emulate that exact testing to ensure `snapd debug seeding` returns clean.

Tested locally using bartender:

1. ran a build using ubuntu-old-fashioned/scripts/ubuntu-bartender with the following command:

bartender --hook-extras-dir ~/dev01/cloudware/cpc_packaging.extra --livecd-rootfs-dir ~/dev01/livecd-rootfs --build-provider aws --aws-keypair-name jchittum-cpc-jenkins --aws-profile default -- --series impish --project ubuntu-cpc --image-target qcow2

This ran a build on aws using local checkouts of livecd-rootfs (with this change) and cpc's extras (which are unused). The `qcow2` build provides the most basic bootable image

2. used qemu to run an instance:

qemu-system-x86_64  -cpu host -machine type=q35,accel=kvm -m 2048 -nographic -snapshot -netdev id=net00,type=user,hostfwd=tcp::2222-:22 -device virtio-net-pci,netdev=net00 -drive if=virtio,format=qcow2,file=livecd.ubuntu-cpc.img  -drive if=virtio,format=raw,file=/home/jchittum/dev01/vmdks/cloud_init_working.iso -drive if=pflash,format=raw,file=/usr/share/OVMF/OVMF_CODE.fd,readonly

This does a UEFI boot with the file produced from the build.

3. ssh'd into the instance and ran `sudo snapd debug seeding`. This returned without the preseed-system-key and seed-restart-system-key

4. ran CPC's cloud-test-framework against the existing instance (thus emulating our current failing case). This came back with no errors

5. Created a custom package of livecd-rootfs with my change

6. request an LP build using the custom package of livecd-rootfs

7. downloaded build and started with qemu (same as #2)

8. ran #3 (ssh + sudo snapd debug seeding) and observered the following:

ubuntu at ubuntu:~$ sudo snap debug seeding
seeded:            true
preseeded:         true
image-preseeding:  26.491s
seed-completion:   1.661s

9. Ran cloud-test-framework, and had it come back green.
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~jchittum/livecd-rootfs:mount-faux-cgroup-for-snapd into livecd-rootfs:ubuntu/master.
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