add required fields for RHSatellite6 credentials

see: https://github.com/ansible/ansible-tower/issues/7467

awx/main/models/credential.py

@@ -861,7 +861,8 @@ def satellite6(cls):
                 'label': 'Password',
                 'type': 'string',
                 'secret': True,
-            }]
+            }],
+            'required': ['host', 'username', 'password'],
         }
     )