Fix upgrade of policy in objects.Cluster

Unknown elements were silently removed on startup. This means that a
software upgrade could result in lost configuration information if
cfgupgrade wasn't run promptly.

Added unit test for Cluster.UpgradeConfig() to cover this case.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>