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
Glance image-list failed when image number exceed DEFAULT_PAGE_SIZE

The root cause of the problem is that DB2 does not allow
an empty string ('') for datetime field. So we set the
default value of datetime as None.

Fixes bug 1245947

Change-Id: Ie372efe70a068024298fbde4574e33e565cc6c9a
Merge "Glance image-list failed when image number exceed DEFAULT_PAGE_SIZE"
Merge "Tests added for glance/cmd/cache_cleaner.py"