Remove calls to policy.enforce from plugin and db logic

Remove calls to policy.enforce from plugin and db logic

Blueprint make-authz-orthogonal

This patch implementes item #2 of the blueprint
Remove calls to policy.enforce when the policy check can be performed
safely at the API level, and modify policy.json to this aim.
This patch does not address enforce calls in the agent scheduler
extension, as that extension is currently not defined as a quantum.v2.api
resource class.
This patch also adds an API-level test case for the provider networks
extension, which was missing in Quantum and was necessary to validate
the API behaviour with the default policy settings.

Change-Id: I1c20a5870279bc5fce4470c90a210eae59675b0c