Enable object caching in cinder REST API requests

Allow the core API to cache resources, such as DB results, so that
extensions can use data already retrieved within the same API request
eliminating additional expensive DB calls.

Loosely based on commit 9f9fbc54e7336da10fc3056bdaca2ec7d01c7f94 from
nova.

Change-Id: If9f49faf7305287c0489ad6209cf19b8bec612cc
Partial-Bug: #1197612
Merge "Enable object caching in cinder REST API requests"