Simplify the diskimage-builder config options in the nodepool config by
removing qemu-img-options from nodepool's configuration. Instead we
hardcode a qcow2 version compat setting of 0.10. This is necessary for
HPCloud and possibly other clouds, and keeps things simple in nodepool.
Change-Id: Icbbb92e22e5cab1332c17291392162d3d86de6b5
caf01c43c2