Set rootwrap_config in cinder.conf sample.

Remove the root_helper=sudo in cinder.conf.sample and
move to the rootwrap_config=/etc/cinder/rootwrap.conf as
the default.

Change-Id: Ib4bacf4d24993cb9506b51523a5ba44fe1f85d18
Merge "Set rootwrap_config in cinder.conf sample."