Set rootwrap_config path to rootwrap.conf

Set rootwrap_config path to rootwrap.conf

Other components set
rootwrap_config = /etc/<component>/rootwrap.conf by default.
This commit brings cinder up-to-date with what other
components are doing by setting
rootwrap_config = /etc/cinder/rootwrap.conf by default.

A couple of test cases needed to be updated to handle the change
from 'None'.

Not counting this as a documentation impact as the documentation I
can find already lists /etc/cinder/rootwrap.conf as the default.

Closes-Bug 1226074

Change-Id: I68dd5fea1895a6365d6c69db0535d081c4a3460e