I think for Juju this is a case where we do different setup on MAAS than on LXD. Specifically w/ the same 2.3.6 and LXD I see:
Cloud-init v. 18.2 running 'modules:config' at Mon, 16 Apr 2018 06:11:54 +0000. Up 16.00 seconds.
+ install -D -m 644 /dev/null /etc/systemd/system/juju-clean-shutdown.service
+ printf %s\n
[Unit]
Description=Stop all network interfaces on shutdown
DefaultDependencies=false
After=final.target

[Service]
Type=oneshot
ExecStart=/sbin/ifdown -a -v --force
StandardOutput=tty
StandardError=tty

[Install]
WantedBy=final.target

+ /bin/systemctl enable /etc/systemd/system/juju-clean-shutdown.service
Created symlink /etc/systemd/system/final.target.wants/juju-clean-shutdown.service → /etc/systemd/system/juju-clean-shutdown.service.
+ install -D -m 644 /dev/null /var/lib/juju/nonce.txt
+ printf %s\n user-admin:bootstrap
Cloud-init v. 18.2 running 'modules:final' at Mon, 16 Apr 2018 06:11:55 +0000. Up 17.00 seconds.
Cloud-init v. 18.2 finished at Mon, 16 Apr 2018 06:11:55 +0000. Datasource DataSourceNoCloud [seed=/var/lib/cloud/seed/nocloud-net][dsmode=net].  Up 17.00 seconds


while on MAAS I had:
Cloud-init v. 18.2 running 'modules:config' at Mon, 16 Apr 2018 05:51:08 +0000. Up 28.17 seconds.
2018-04-16 05:51:08,730 - util.py[WARNING]: Running module apt-configure (<module 'cloudinit.config.cc_apt_configure' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_apt_configure.py'>) failed
2018-04-16 05:51:08,930 - schema.py[WARNING]: Invalid config:
runcmd: ['set -xe', "mkdir -p '/var/lib/juju'\ncat > '/var/lib/juju/MAASmachine.txt' << 'EOF'\n'hostname: nuc7\n'\nEOF\nchmod 0755 '/var/lib/juju/MAASmachine.txt'", 'set -xe', "install -D -m 644 /dev/null '/etc/systemd/system/juju-clean-shutdown.service'", "printf '%s\\n' '\n[Unit]\nDescription=Stop all network interfaces on shutdown\nDefaultDependencies=false\nAfter=final.target\n\n[Service]\nType=oneshot\nExecStart=/sbin/ifdown -a -v --force\nStandardOutput=tty\nStandardError=tty\n\n[Install]\nWantedBy=final.target\n' > '/etc/systemd/system/juju-clean-shutdown.service'", "/bin/systemctl enable '/etc/systemd/system/juju-clean-shutdown.service'", "install -D -m 644 /dev/null '/var/lib/juju/nonce.txt'", "printf '%s\\n' 'user-admin:bootstrap' > '/var/lib/juju/nonce.txt'"] has non-unique elements
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Suggested packages:
  ifupdown
The following NEW packages will be installed:
  bridge-utils
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+ mkdir -p /var/lib/juju
+ cat
+ chmod 0755 /var/lib/juju/MAASmachine.txt
+ set -xe
+ install -D -m 644 /dev/null /etc/systemd/system/juju-clean-shutdown.service
+ printf %s\n
[Unit]
Description=Stop all network interfaces on shutdown
DefaultDependencies=false
After=final.target

[Service]
Type=oneshot
ExecStart=/sbin/ifdown -a -v --force
StandardOutput=tty
StandardError=tty

[Install]
WantedBy=final.target

+ /bin/systemctl enable /etc/systemd/system/juju-clean-shutdown.service
Created symlink /etc/systemd/system/final.target.wants/juju-clean-shutdown.service → /etc/systemd/system/juju-clean-shutdown.service.
+ install -D -m 644 /dev/null /var/lib/juju/nonce.txt
+ printf %s\n user-admin:bootstrap
Cloud-init v. 18.2 running 'modules:final' at Mon, 16 Apr 2018 05:51:09 +0000. Up 29.35 seconds.
Cloud-init v. 18.2 finished at Mon, 16 Apr 2018 05:51:15 +0000. Datasource DataSourceMAAS [http://10.0.0.1:5240/MAAS/metadata/].  Up 35.69 seconds