Juju controller not selected during appliance upgrade

[Alexis Janon]

When upgrading the appliance, the process may fail as the juju status command does not specify the juju controller explicitly. From the upgrade logs:

++ /usr/bin/sudo -u ubuntu -H --preserve-env=https_proxy,http_proxy,no_proxy /snap/anbox-cloud-appliance/578/bin/juju status --format=json lxd-integrator
++ python3 -c 'import sys, json; print(json.load(sys.stdin)['\''applications'\'']['\''lxd-integrator'\'']['\''charm-rev'\''])'
ERROR No selected controller.