Libvirt-Enable support for discard option for disk device

Support setting discard option for instance's disk.
If discard option is enabled, unused/freed blocks can be return back to the
storage device.

blueprint libvirt-disk-discard-option
DocImpact: Add a new configuration in nova.conf

Change-Id: Ic87b9348a2e441d5df0b468bb88ec6e77f8b4ce2
Merge "Libvirt-Enable support for discard option for disk device"