Add the StorPool block storage driver.

Add the StorPool block storage driver.

StorPool is distributed data storage software running on standard x86
servers.  StorPool aggregates the performance and capacity of all drives
into a shared pool of storage distributed among the servers.  Within
this storage pool the user creates thin-provisioned volumes that are
exposed to the clients as block devices.  StorPool consists of two parts
wrapped in one package - a server and a client.  The StorPool server
allows a hypervisor to act as a storage node, while the StorPool client
allows a hypervisor node to access the storage pool and act as a compute
node.  In OpenStack terms the StorPool solution allows each hypervisor
node to be both a storage and a compute node simultaneously.

To make full use of StorPool's native network communication protocol,
the Nova compute nodes will need to use the StorPool libvirt volume
attachment driver, nova.virt.libvirt.storpool.LibvirtStorPoolVolumeDriver.

DocImpact
Change-Id: I64adbca724a52771b68e4838749f8896e6e56019
Implements: blueprint storpool-block-driver