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

Closes-Bug: #1330532

Conflicts:
	neutron/wsgi.py

Change-Id: I95581dc5dffde6168c68d9f04cf8bdfcdb773ba8
(cherry picked from commit 09856a0cf63052045b3d9f4c8d2a5aa920c7c935)
(cherry picked from commit e83202ba54c47dc4754a38a2911331e34e298c3c)