Added line to systemd config template to start Nebula before sshd (#317)

During shutdown, this will keep Nebula alive until after sshd is finished. This cleanly terminates ssh clients accessing a server over a Nebula tunnel.

examples/service_scripts/nebula.service

@@ -2,6 +2,7 @@
 Description=nebula
 Wants=basic.target
 After=basic.target network.target
+Before=sshd.service
 
 [Service]
 SyslogIdentifier=nebula