Implement rbd disk template

Introduce the rbd disk template, which handles provisioning and
management of instance disks as block devices mapped to rbd volumes
on a RADOS cluster.

The rbd template is of type DTS_EXT_MIRROR, meaning rbd-based
instances can failover or migrate to any node, without
primary-secondary limitations.

Use of this template requires the rbd kernel driver and rbd tool
on all VM-capable nodes, as well as an operational RADOS cluster,
accessible by them.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>