unattended-upgrades must be disabled for machines with collocated charms before upgrading

[samuelallan72]

According to https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/yoga/upgrade-openstack.html#disable-unattended-upgrades :

When performing a service upgrade on a cloud node that hosts multiple principal charms (e.g. nova-compute and ceph-osd), ensure that unattended-upgrades is disabled on the underlying machine for the duration of the upgrade process. This is to prevent the other services from being upgraded outside of Juju’s control. On a cloud node run:

sudo dpkg-reconfigure -plow unattended-upgrades

Then after all upgrades are complete: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/yoga/upgrade-openstack.html#re-enable-unattended-upgrades

In a previous step, unattended-upgrades were disabled on those cloud nodes that hosted multiple principal charms. Once such a node has had all of its services upgraded, unattended-upgrades should be re-enabled:

sudo dpkg-reconfigure -plow unattended-upgrades

charmed-openstack-upgrader currently does not do this procedure. We should add support for this, since it's a required step for upgrades.