vm_devbox: changes to service file so systemd can manage the service

[e-carlin]

• [x] remove PrivateTmp (this was causing many problems)
• [x] Type=forking (the vm process forks and stays running after vagrant up exits)
• [x] vagrant reload after setting forwarded_port (need to reload for change to take)
• [x] execStop (needs to be an abspath and needs to be in /srv/vm_devbox_user. Just use a stop script and call it)
• [x] TimeoutStartSec=10min. With Type=forking the timeout is at play