When os-brick starts using privsep, it will need to know how to invoke
its privileged half. Amazingly the name of the rootwrap executable
isn't anywhere else in the config, so the privsep default uses just
"sudo" (no rootwrap).

We need to either:

1. set the privsep command line to use cinder-rootwrap in
cinder.conf (and similar in other configs), or

2. add the privsep-helper line to sudoers and bypass rootwrap entirely.

This change implements (1) for devstack/cinder and is similar to the
corresponding nova change in I90dc41bc77993bd83b80c92286e015e14f290b45

Change-Id: I8a0b1728cc66c4861f69623b1b16b1f759b57b25