Don't allow to change login/password for VPN from the subscription admin view

coin/vpn/admin.py

@@ -12,7 +12,8 @@ from coin.utils import delete_selected
 class VPNConfigurationInline(admin.StackedInline):
     model = VPNConfiguration
     # fk_name = 'offersubscription'
-    readonly_fields = ['configuration_ptr']
+    exclude = ('password',)
+    readonly_fields = ['configuration_ptr', 'login']
 
 
 class VPNConfigurationAdmin(ConfigurationAdminFormMixin, PolymorphicChildModelAdmin):