Cleanup cinder-rootwrap support

cinder_rootwrap support in devstack handled a number of now-abandoned
use cases:

- no $CINDER_DIR/etc/cinder/rootwrap.d (old-style rootwrap)
- using oslo-rootwrap instead of cinder-rootwrap (abandoned experiment)

This change removes unused code paths and aligns
configure_cinder_rootwrap() with configure_nova_rootwrap().

Change-Id: I387808dae0e064cc9c894c74ab78e86124f08dd2
Merge "Cleanup cinder-rootwrap support"