Update the cluster serial_no on certain operations

This patch adds update of the cluster serial number for:
  - add/remove node (as the cluster's node list is changed)
  - add/remove/rename instance (as the cluster's instance list is changed)
  - change the volume group name

The rule for updating this attribute is when cluster-wide properties are
changed, but not individual node/instance ones.

There are other remaining cases to handle, pending on the ssconf
changes.

Reviewed-by: ultrotter