add some tests for values

cannot use __setitem__ to set value for an option
in append, len should not greater than master option

test/test_option_consistency.py

@@ -133,6 +133,7 @@ def test_consistency_ip_netmask_multi_master():
     c.b = ['255.255.255.255']
     c.b = ['255.255.255.0']
     raises(ValueError, "c.a = ['192.168.1.0']")
+    c.a = ['192.168.1.2', '192.168.1.3']
 
 
 def test_consistency_network_netmask_multi_master():