Enable use of Glance driver from nova

This switches nova to using the driver architecture to access
glance V1. Also some associated xenapi driver changes moving
the image updates out of the actual plugin and into the calling
code. The plugin is now ONLY responsible for upload / download.
This will work better with Glance V2.

Implements bp use-glance-v2-api

Change-Id: I8882f0625bcd9d02b2a99edd148566af024b8818