Fix EZP-22025: use a real cluster file scope

'true' was passed as the scope when storing the file to cluster.
The scope is now set to image, so that expired cache files can
easily be purged with system scripts.
Merge pull request #18 from ezsystems/fix/EZP-22025-ezie_tmp_files_scope

Fix EZP-22025: use a real cluster file scope