Cells: Reduce the create_image call depth for cells

This patch reduces the create image call depth (for snapshot/backup) by
returning the basic image details to the user before we call down to
the child cell.  The image entry is created in glance immediately, and
the child cell will finish up the rest.

Implements blueprint nova-compute-cells

Change-Id: I048cc532614c602f3aa6b539fb51e951174e4598
Merge "Cells: Reduce the create_image call depth for cells"