fix(computes): VM ownership survives a move or a short absence

The sync deleted the Instance of every VM missing from its compute, and
its owners and rights went with it. A VM moved with virsh, or undefined
and defined again, came back with no owner.

Now, when a VM disappears from a compute:
- if the same UUID is recorded on another compute, it moved: its owners
  and template flag go there;
- otherwise they are kept in an InstanceTombstone and restored when the
  UUID appears again within INSTANCE_OWNERSHIP_RETENTION_DAYS (30).
A UUID recorded on two computes at the same time is a copy and gets no
owners. VMs deleted through WebVirtCloud leave no tombstone.

webvirtcloud/settings.py.template

@@ -287,6 +287,8 @@ LIBVIRT_KEEPALIVE_COUNT = 5
 
 ALLOW_EMPTY_PASSWORD = False
 NEW_USER_DEFAULT_INSTANCES = []
+# Days the owners of a VM that vanished from its compute are kept, in case it comes back
+INSTANCE_OWNERSHIP_RETENTION_DAYS = 30
 SHOW_PROFILE_EDIT_PASSWORD = True