Support Swift Temporary URLs for Glance Images

Swift allows you to create signed URLs which expire after some amount of time that don't require auth tokens. If Glance is using Swift as a backend, this will allow the conductor to create a URL to send to the agent that doesn't require the agent to hold auth tokens or passwords of any kind. This will require Glance v2 and a set of config options to specify the Swift API version (swift_api_version, defaults to 'v1'), auth user (swift_account), container (swift_container, defaults to 'glance'), and Swift endpoint (swift_endpoint_url) that the images are hosted in.