Relocate DATADIR to backends

It simply makes sense that the definition of DATADIR belongs to
backends. After all, some of them may not even have any.

Coincidentially, a few unnecessary imports are dropped.

By the way, on the object server side, diskfile.py provides DATADIR
in the same way already.

Change-Id: I60bfd522c77c4a0ee13697a2e31141777c7e2398
Merge "Relocate DATADIR to backends"