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 allow a deploy driver to download an image
from Glance without requiring an auth_token, which gives access more than
just Glance. This requires a set of config options: the Glance endpoint URL,
a shared secret temporary key, and the account configured in Glance to talk
to Swift with, which owns the container images are stored in.

A later patch will explore using the direct_url in Glance to simplify the
config options.

We/I need to add a note in the docs about Swift being a possible dependency
for Ironic deploys using IPA, along with how to set the Temp URL key.

Swift performance concerns will be addressed in this blueprint:
https://blueprints.launchpad.net/ironic/+spec/improve-swift-agent-downloads

SecurityImpact
DocImpact

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