Tests added for glance/cmd/cache_pruner.py

Related to blueprint glance-cmd+unittests

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

Change-Id: I7d7b182260a407aed1288a5d136e03cbe029523f
Merge "Tests added for glance/cmd/cache_pruner.py"