cloud-init fails when launching images:ubuntu/16.04/cloud VM instances

Steps to reproduce:
```
import pycloudlib
lxdvm = pycloudlib.LXDVirtualMachine(tag="bar")
image = "images:ubuntu/16.04/cloud"
i = lxdvm.launch(image)
```
The VM starts fine and can be accessed via `lxc exec bash` (thus via lxd-agent), but cloud-init is in a bad state:
```
root@patient-stud:~# cloud-init status
status: error
```
The reason is:
```
2021-02-22 11:01:47,963 - helpers.py[DEBUG]: Running config-scripts-per-once using lock ()
2021-02-22 11:01:47,964 - subp.py[DEBUG]: Running command ['/var/lib/cloud/scripts/per-once/setup-lxc.sh'] with allowed return codes [0] (shell=False, capture=False)
2021-02-22 11:01:48,165 - subp.py[DEBUG]: Unexpected error while running command.
Command: ['/var/lib/cloud/scripts/per-once/setup-lxc.sh']
Exit code: 5
Reason: -
Stdout: -
Stderr: -
2021-02-22 11:01:48,166 - cc_scripts_per_once.py[WARNING]: Failed to run module scripts-per-once (per-once in /var/lib/cloud/scripts/per-once)
2021-02-22 11:01:48,166 - handlers.py[DEBUG]: finish: modules-final/config-scripts-per-once: FAIL: running config-scripts-per-once with frequency once
2021-02-22 11:01:48,167 - util.py[WARNING]: Running module scripts-per-once () failed
2021-02-22 11:01:48,167 - util.py[DEBUG]: Running module scripts-per-once () failed
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 849, in _run_modules
freq=freq)
File "/usr/lib/python3/dist-packages/cloudinit/cloud.py", line 54, in run
return self._runners.run(name, functor, args, freq, clear_on_fail)
File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 186, in run
return (True, results)
File "/usr/lib/python3.5/contextlib.py", line 77, in __exit__
self.gen.throw(type, value, traceback)
File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 74, in lock
yield self._acquire(name, freq)
File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 185, in run
results = functor(*args)
File "/usr/lib/python3/dist-packages/cloudinit/config/cc_scripts_per_once.py", line 42, in handle
subp.runparts(runparts_path)
File "/usr/lib/python3/dist-packages/cloudinit/subp.py", line 386, in runparts
(len(failed), ",".join(failed), len(attempted)))
RuntimeError: Runparts: 1 failures (setup-lxc.sh) in 1 attempted commands
```
This doesn't happen when manually launching an instance using the same image, e.g.
```
lxc launch images:ubuntu/16.04/cloud test-x-vm --vm
```
in this case:
```
root@test-x-vm:~# cloud-init status
status: done
```
This was done on the same host machine and the problem is can be reproduced consistently.