Don't fail if volume has no image metadata

In the same way that nova shouldn't fail when booting an instance if the
image has no metadata, when booting from a volume it should accept the
fact that it has no image_metadata and return an empty dict.

Fixes bug #1220814

Change-Id: Idc0ff2106415a0662e57021e918dc5c6bf9a9a92
Merge "Don't fail if volume has no image metadata"