Add GlusterFS volume driver

Add GlusterFS volume driver

This driver enables use of GlusterFS in a similar fashion
as the NFS driver.  It supports basic volume operations,
and like NFS, does not support snapshot/clone.

To enable, set volume_driver to
 cinder.volume.drivers.glusterfs.GlusterfsDriver

Note that this requires a Nova libvirt GlusterFS driver
as well.

Adds config options: glusterfs_shares_config,
glusterfs_mount_point_base, glusterfs_disk_util, and
glusterfs_sparsed_volumes.

DocImpact

Change-Id: I3dd4018f0cb4db48348728ca66bae7918309bb32