Update GlusterFS driver

Since https://github.com/openstack/cinder/commit/e997f8b06ecd4ed0a13c557d621db69eb68f2f3d
got merged, glusterfs_disk_util is now removed.
This patchs aims to raise an error when using it.

Change-Id: Ic5b9b0a742482d6b7380cb59fbec6b490a206779
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Update GlusterFS driver"