Code review comment for ~morphis/netplan/+git/netplan:nm-snap-support

Reworked things a bit as we need to use the nmcli binary from the snap too. With that it makes things even harder to just abstract the snap use through an Alias= line in the systemd service unit as we also need to use the right nmcli binary which is "network-manager.nmcli" when it comes from the snap and just "nmcli" when its on a Ubuntu classic system.