Use image from the api in run_instance, if present

When booting an instance, the API layer already fetches the image
from glance and passes it to the compute manager. Right now, that
info is ignored and the compute node has fetches the image metadata
again from glance if booting from an image. This is a problem when
booting from a volume, because most of the properties that the volume
keeps when it has been created from an image won't be considered.

With this patch, the image metadata passed to run_instance from the API
will be used if present instead of asking glance again.

Change-Id: I4680d9d309d7a09f64d52ef0fb105282ace79e5f
Closes-Bug: 1281973
Merge "Use image from the api in run_instance, if present"