Sync service and systemd modules from oslo-incubator

This patch make systemd know when neutron-service was started. This is
needed in HA environment, previously systemd returned success even
before neutron-server was able to handle requests.

Current oslo-incubator commit on HEAD:
b7ad6ddab8b1d61bf4f52ccaa461a9d68809747b

Implements: blueprint service-readiness
Change-Id: Ic9e4abd11b614a896fbd7454b9a604a69a248d0f
Call policy.init() once per API request

This patch changes the policy engine behaviour and the API base
controller in order to ensure policy.init is invoked only once
for each API request.
This will avoid issues arising from policy file updates during
API processing and speed up response generation for list operations,
by about 5%.

This patch also removes an obsolete TODO comment.

Change-Id: I108ebd26fccdea19cb00959f70d87c3bc1587df9
Closes-Bug: 1302611
Merge "Sync service and systemd modules from oslo-incubator"
Merge "Call policy.init() once per API request"