Make possible to store snapshots not in /tmp directory
Make possible to store snapshots not in /tmp directory

Add option libvirt_snapshots_directory which specifies path
where snapshots will be stored before libvirt driver uploads
them to the image service.

VM disk snapshot can be quite large,
so keeping them in /tmp is not desirable. Fedora 18 will move /tmp
to tmpfs in RAM, so snapshot functionality will become unusable with
current nova behavior.

Default path for snapshots directory is set to $instances_path/snapshots
in this patch. In case, when shared filesystem is used to hold instances,
option value can be adjusted to improve performance.

Change-Id: I9458a2a26a619a01fa1ddb6955c0220ca747e64d

n0ano/gantt,n0ano/gantt