disable direct mounting of qcow2 images by default

* nova/virt/disk/api.py: Set the default for the
'resize_fs_using_block_device' option to False,
so that we don't enable this functionality because:
1. It's not needed for newer images that resize themselves.
2. It enables direct mounting of images which might otherwise
   be disabled for security reasons.
3. It's not supported on systems without the nbd module.
4. It gives noisy diagnostics on such systems.

Fixes bug: 1212734

Change-Id: Ifbd687eae46606176ea196888eeffdde08559c01
Merge "disable direct mounting of qcow2 images by default"