explicitly set volumes_dir in cinder.conf

Fixes: bug #1269475
Implements: blueprint centos-rhel-for-block

On rhel systems, /usr/share/cinder/cinder-dist.conf sets:

volumes_dir=/etc/cinder/volumes

In order set this back to the default cleanly, we need to explicitly set it in
cinder.conf which will take precedence over the setting in
cinder-dist.conf

Change-Id: Id71efd10b0cbd27b72dfe8ec72c51cde70593b3e