FEATURE: PCI-1526 - LVM Thin volumes support added

To allow OpenStack use LVM thin volumes, the following changes have been made:
1. new configuration options added:
    libvirt_thin_logical_volumes - boolean, default False
    libvirt_thin_logical_volumes_overcommit - float, default 1.5 (150%)
2. nova libvirt utils changed to support the thin volumes and thin pool creation