oslo.privsep should be used by default. We should be able to use
processutils.execute when os-brick is used outside Nova or Cinder.
E.g.: python-brick-cinderclient-ext should be used with sudo or run
as root without any helpers like oslo.privsep or oslo.rootwrap.
Depends-On: I489193c18ae6603f0bc750c7b63e1dd0b4a71e28
Change-Id: I5a10574cb91794f978eda2aa2e2e65dc92ca56b4