Support adding multiple additional disks to gluster machines.

You can easily configure this by settings the disks integer in the
puppet-gluster.yaml file or with the --gluster-disks= parameter.

If this is set to the default of zero (0) then no disks are added.

This patch depends on:
https://github.com/pradels/vagrant-libvirt/issues/162
Set the libvirt driver to use accelerated kvm.

This wasn't necessary in previous versions of vagrant-libvirt because
this option specified the uri, and the domain type defaulted to kvm. In
newer releases, you have to specify this for performance, otherwise it
will default to qemu software only virtualization. This depends on:
https://github.com/pradels/vagrant-libvirt/pull/182