Tests added for glance/cmd/cache_cleaner.py

Integration test test_cache_cleaner_main added to cover
chain of calls between glance/cmd/cache_cleaner.py and glance/image_cache
module.
Unit test test_cache_cleaner_main_runtime_exception_handling added to
cover exception handling for main() method of cache_cleaner.py

Change-Id: Ic05f35dd3f0a93d4331ccb5e470cdb71d9eab605
Merge "Tests added for glance/cmd/cache_cleaner.py"