GlusterFS: Use image_utils for tempfile creation

Besides the aesthetical aspect of replacing ad-hoc code
with an utility function, it's more correct
to perform image conversions in CONF.image_conversion_dir,
which is taken care by the image_utils.temporary_file()
context guard.

Change-Id: Ied5607156f0ab1f1f8cfc889d5b57f3486d6b407
Merge "GlusterFS: Use image_utils for tempfile creation"