Adding swift temp url support

This patch will allow properly configured Glance servers to return a temporary
URL for an object hosted on Swift. It will require Glance to use Swift
as its backend. A temporary URL will let the agent download an image from
Glance without requiring an auth_token, which gives access more than just
Glance. This is a security concern.

Implements: blueprint swift-temp-urls
Change-Id: I10e4784eee63e8edc9ba30a9c5004a08aa3a6d8e