LXD upgrade charm from bundle

[SimonRichardson]

Description of change

The following upgrades a LXD charm from a bundle charm.
Also fixes some potential issues with subordinate units.

QA steps

mkdir /tmp/test-run
mkdir /tmp/artifacts
export JUJU_HOME=/tmp/test-run
export JUJU_REPOSITORY=$GOPATH/src/github.com/juju/juju/acceptancetests/repository
vim $JUJU_HOME/environments.yaml

Local LXD environments.yaml

environments:
    lxd:
        type: lxd
        test-mode: true
        default-series: bionic

Run the following (substitute the right arg):

cd acceptancetests
./assess_upgrade_lxd_profile.py

[SimonRichardson]

https://trello.com/c/By5Z8XPK/1107-1813799-juju-upgrade-profile-with-subordinates

[SimonRichardson]

Closing this as #9776 supersedes it.