juju-2.1-beta3 cannot add LXD container after host machine has rebooted

TL;DR

The root cause, I believe, is:

  https://github.com/juju/juju/pull/6684

# Reproducible steps:

$ juju version
2.1-beta3-xenial-amd64

$ juju bootstrap maas ...

$ juju status
Model Controller Cloud/Region Version
controller maas21-daily maas21-daily 2.1-beta3.1

Machine State DNS Inst id Series AZ
0 started 172.16.101.81 gtr46e xenial default

$ juju add-machine lxd:1

$ juju ssh 0/lxd/0 uname -n
juju-81c46c-0-lxd-0

# REBOOT THE HOST MACHINE
$ juju ssh 0 sudo reboot
# Wait until machine comes up

$ juju status
Model Controller Cloud/Region Version
controller maas21-daily maas21-daily 2.1-beta3.1

Machine State DNS Inst id Series AZ
0 started 172.16.101.81 gtr46e xenial default
0/lxd/0 started 172.16.101.82 juju-81c46c-0-lxd-0 xenial

$ juju add-machine lxd:0
# Machine ends up pending state...

$ juju status
0 started 172.16.101.81 gtr46e xenial default
0/lxd/0 started 172.16.101.82 juju-81c46c-0-lxd-0 xenial
0/lxd/1 pending pending xenial

Looking in the logs we see the following repeated:

2016-12-22 21:00:16 INFO juju.worker runner.go:262 stopped "0-container-watcher", err: worker "0-container-watcher" exited: setting up container dependencies on host machine: while running lxd init --auto: LXD init cannot be used at this time.
However if all you want to do is reconfigure the network,
you can still do so by running "sudo dpkg-reconfigure -p medium lxd"

error: You have existing containers or images. lxd init requires an empty LXD.
: exit status 1
2016-12-22 21:00:16 DEBUG juju.worker runner.go:190 "0-container-watcher" done: worker "0-container-watcher" exited: setting up container dependencies on host machine: while running lxd init --auto: LXD init cannot be used at this time.
However if all you want to do is reconfigure the network,
you can still do so by running "sudo dpkg-reconfigure -p medium lxd"

error: You have existing containers or images. lxd init requires an empty LXD.
: exit status 1

The root cause, I believe is this PR:

  https://github.com/juju/juju/pull/6684