Direct image upload to Swift
Give Nova the ability to upload image data directly to Swift.
Existing behavior (as of Folsom)
Currently, Nova uses Glance to upload image data. This is inefficient as the data goes from Nova to Glance to Swift.
When the setting `direct_image_upload` is enabled in Nova configuration, Nova will upload image data directly to Swift.
- We want to respect both single- and multi-tenant storage modes in Swift.
- For single-tenant storage, need to have the single tenant's credentials available to the code doing the upload.