improve performance of uploading ISO
Adding a search criteria to filter out units as need to improve performance of "find_repo_content_units" in pulp_rpm/plugins/importers/iso/importer.py
We suffered serious performance issue to uploading units (ISO) to a repository when data volume get larger and larger. After applying the change, performance improved. In addition, some our internal function tests had been done, it's pass. We uses pulp 2.15.