Host shutdown should trigger snap stop

[Matt Verran]

As per the tutorials on https://ubuntu.com/tutorials/tear-down-your-openstack-lab-environment#2-stop-and-sunbeam when the host machine (vm, multipass, maas, etc) is shutdown, it should cleanly stop.

This means on restart mysql isn't in a mess and having to recover, allowing vault to start if enabled, and from there the rest of the services.

Something like:-

# /etc/systemd/system/openstack_shutdown.service
[Unit]
Description=Shutdown Openstack cleanly...
After=network.target
Requires=network.target

[Service]
Type=simple
ExecStart=/bin/true
ExecStop=sudo snap stop microk8s;sudo snap stop openstack-hypervisor
TimeoutStopSec=infinity

[Install]
WantedBy=multi-user.target