Gluster: add GlusterVolume class

In order for Ganeti to manage Gluster volumes, Ganeti needs to know
what a Gluster volume is to begin with. This class models one.
Gluster: add configure switch for mount point location

Allow system administrators to specify where the mount should happen.
Gluster: automatically un/mount gluster volumes

Allow Ganeti to manage mounts of Gluster volumes autonomously.
Gluster: FIXME: add userspace access support

Add support for the QEMU gluster: protocol
Gluster: add requirements to install.rst
Gluster: announce in NEWS

Add the relevant line to NEWS