Call _destroy_metadata_proxy from _destroy_router_namespaces

Refactor _spawn/destroy_metadata_proxy so that it can be called
with only the namespace and the router_id.

Change-Id: Id1c33b22c7c3bd35c54a7c9ad419831bfed8746b
Closes-Bug: #1252856
(cherry picked from commit 07d597079781967f5a149f1812ddca3897fa49d9)
Related-Bug: #1175695
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 _destroy_metadata_proxy from _destroy_router_namespaces" into stable/havana
Merge "Call policy.init() once per API request" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable