Put FakeTransferClient and FakeServiceClient classes
into FakeVolumeClient class, I think we need not to
create FakeClient classes for every objects, we can use
only FakeVolumeClient for all volume objects. This can
reduce repetition, unified codes and make codes look
cleaner and easier.
Change-Id: I2729ef0e80166f4d49ccd9a48b653e2f215e3bfe