Make libvirt_images_type configurable

This setting allows changing the way libvirt stores ephemeral disk
images.  Possible settings are lvm, rbd, qcow2, raw and default.

When you pick "default", nova will use qcow2 when use_cow_images is
true. This cookbook does not handle that value, its default is true, so
default and qcow2 actually means the same thing here.  (There's no harm
done, I think.)

It includes additions to the README, specs and small cosmetic changes to
spec/nova-common_spec.rb.

Change-Id: If67029b1392f5657cde5da12702f757cb3c61805
Merge "Make libvirt_images_type configurable"