Deprecate root_helper in favor of rootwrap_config

Deprecate root_helper in favor of rootwrap_config

Align with recent changes in nova-rootwrap by marking the
root_helper option deprecated and introduce usage of the
rootwrap_config option instead. The root_helper option will still
fully be supported in Folsom, but will be removed in Grizzly.

Transition notes: you should replace:
root_helper=sudo cinder-rootwrap /etc/cinder/rootwrap.conf

by:
rootwrap_config=/etc/cinder/rootwrap.conf

Change-Id: I22a6d2bdee6ad2c5ad587ceec574cec4b2887f22