module: fix default value for dhcp4_send_hostname

We should not set "dhcp4_send_hostname" unless it is explicitly
present in the configuration. That is because initscripts don't
support this parameter and issue a warning.

library/network_connections.py

@@ -619,9 +619,7 @@ class ArgValidator_DictIP(ArgValidatorDict):
             result['dhcp4'] = result['dhcp4_send_hostname'] is not None or not any([a for a in result['address'] if a['is_v4']])
         if result['auto6'] is None:
             result['auto6'] = not any([a for a in result['address'] if not a['is_v4']])
-        if result['dhcp4_send_hostname'] is None:
-            result['dhcp4_send_hostname'] = False
-        else:
+        if result['dhcp4_send_hostname'] is not None:
             if not result['dhcp4']:
                 raise ValidationError(name, '"dhcp4_send_hostname" is only valid if "dhcp4" is enabled')
         return result

library/test_network_connections.py

@@ -272,7 +272,7 @@ class TestValidator(unittest.TestCase):
                         'dns_search': [ 'aa', 'bb' ],
                         'route_metric6': None,
                         'ip_is_present': True,
-                        'dhcp4_send_hostname': False,
+                        'dhcp4_send_hostname': None,
                     },
                     'mac': None,
                     'master': None,