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
(cherry picked from commit 1d41ba485f060a4d6dbe0767845391f955247b08)
Merge "Call policy.init() once per API request" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse