Some public functions in the diskfile manager expect or return full
file paths. It implies a filesystem diskfile implementation.
To make it easier to plug alternate diskfile implementations, patch
functions to take more generic arguments.
This commit changes DiskFileManager yield_hashes() returned values
from:
- object_path, object_hash, timestamps
to:
- object_hash, timestamps
object_path was not used by any caller.
Change-Id: I914fb1ec8ce7c9c26d22e1d07f03bd03f4504176