Notify systemd when starting Neutron server

systemd is notified only when Neutron is started with multiple api
workers. This patch adds systemd.notify_once() call when starting
Neutron server with api_workers=0

Change-Id: I95581dc5dffde6168c68d9f04cf8bdfcdb773ba8
Closes-Bug: #1330532
Merge "Notify systemd when starting Neutron server"