doc: HA Juju Controller on LXD instructions aren't working

```
sudo groupadd bootstrap
sudo useradd -m -g bootstrap -s /bin/bash bootstrap
sudo usermod -a -G lxd,sudo bootstrap
sudo passwd bootstrap
sudo -i
su bootstrap
cd
...
juju clouds
```

These instructions don't work anymore:

```
ubuntu@node02:~$ sudo groupadd bootstrap
ubuntu@node02:~$ sudo useradd -m -g bootstrap -s /bin/bash bootstrap
ubuntu@node02:~$ sudo usermod -a -G lxd,sudo bootstrap
ubuntu@node02:~$ sudo passwd bootstrap
New password:
Retype new password:
passwd: password updated successfully
ubuntu@node02:~$ sudo -i
root@node02:~# su bootstrap
To run a command as administrator (user "root"), use "sudo ".
See "man sudo_root" for details.

bootstrap@node02:/root$ cd
bootstrap@node02:~$ juju clouds
/user.slice/user-1000.slice/session-3.scope is not a snap cgroup for tag snap.juju.juju
```

If I instead do `ssh bootstrap@node02` juju commands do work.

Possibly related: [LP#2075560](https://bugs.launchpad.net/snapd/+bug/2075560)

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Document: how-to/misc/bootstrap-highly-available-juju-controller-on-top-of-a-lxd-cluster.rst