Update HACKING.rst with regard to mock usage

Add a sentence about using mock rather than mox for unit tests.

Change-Id: If3c175d986f3ebed6d949df5c473e0cf9177ef6e
Merge "Update HACKING.rst with regard to mock usage"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f