Standardization of nova.image.API.download

Brings the wacky download method into the new nova.image.API object and
removes calls to nova.image.glance throughout the virt drivers that were
using various call signatures of glance.ImageService.download. Since
some of the wrapper objects that fetched image bits from Glance were
using the get and update methods of the former
nova.image.glance.GlanceImageService object, those calls were updated to
instead call the new, standardized nova.image.API methods of the same
name and call signature.

The final patch in this series will refactor the remaining direct use of
the nova.image.glance module, which are calls to
glance.generate_image_url() and glance.generate_glance_url().

Change-Id: Ib3a85d0875c06c4e54e4211bc2fb8ce3fe16161d
Partially-implements blueprint: standardize-nova-image
Merge "Standardization of nova.image.API.download"