Improve use of temporary_file and temporary_dir

Improve use of temporary_file and temporary_dir

cinder/image/image_utils.py contains the functions temporary_file and
temporary_dir for use with handling temporary data.

temporary_file automatically creates files in CONF.image_conversion_dir
and will create CONF.image_conversion_dir if required. This commit
removes duplicate code that checks for/creates the dir before calling
temporary_file. This commit also replaces code that duplicates the
functionality of temporary_file with temporary_file.

temporary_dir requires that CONF.image_conversion_dir exists. This
commit adds a check to the function that creates it if it is missing.

Change-Id: Idecba85e19a0fe16dfc9f840913857137bfeee1b