GPFS convert glance image to raw only when needed

Modify _gpfs_fetch_to_raw to check image format.  If already in raw
format, no need to call convert_image.  In that case, simply rename
to destination name.

Change-Id: I197d99194f93c02ce3c501de25433d89b23e6a35
Fixes: bug #1201676
Merge "GPFS convert glance image to raw only when needed"