Consider downloading a valid status for CinderVolume

"downloading" should be considered a in-progress state while creating a volume
from an image. Because Cinder will first create a volume with specified size
and set its status to downloading, then start fetching the image from glance
and copying it to the volume.

Fixes bug #1222676

Change-Id: I573440f3184c3d47df0d7e78146b0ed030606d86
Merge "Consider downloading a valid status for CinderVolume"