Make the storage and cdn implementations mockable

[jweyrich]

Define interfaces for implementations that support the dry-run flag so we can mock those implementations and no longer require the dry-run flag to be passed around. A dummy example to explain:

if dry_run:
  cdn_impl = MockCdnImpl()
  storage_impl = MockStorageImpl()
else:
  cdn_impl = ReadCdnImpl()
  storage_impl = RealStorageImpl()
  
cdn_impl.do_something()
storage_impl.do_something()