Support oslo-rootwrap in lib/cinder

Make lib/cinder support both cinder-rootwrap (current case) and
oslo-rootwrap (future case) to handle the Cinder transition towards
oslo-rootwrap usage peacefully.

Related blueprint:
https://blueprints.launchpad.net/cinder/+spec/cinder-oslo-rootwrap

Change-Id: I663986304bd74cb6d72d51c553540fb5f9db1d1d
Merge "Support oslo-rootwrap in lib/cinder"