Ensure unused ports return to the free port pool

Ensure ports previously allocated by calling ConfigWriter's AllocatePort() are
returned to the pool of free ports when no longer needed:

 * Return the network_port of an instance when it is removed
 * Return the port used by a DRBD-based disk when it is removed

Signed-off-by: Vangelis Koukis <vkoukis@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>