Fix infinite recursion on calculated master of multi variable

There was a infinite recursion between option.Multi.append() and
config.Config._getattr() through config.Config.fill_multi() when
calculating the master:

- config.Config._getattr()
-> config.Config.fill_multi()
-> option.Multi.__init__()
-> option.Multi.append()
-> config.Config._getattr()

* tiramisu/option.py (Multi.append): do not pass by
  config.Config._getattr().

* tiramisu/option.py (Multi.__init__): do not catch exception when
  appending.

Ref: #4799 @8h

tiramisu/option.py

@@ -58,10 +58,7 @@ class Multi(list):
             super(Multi, self).__init__(lst)
             # we add the slaves without modifying the master
             for l in lst:
-                try:
-                    self.append(l, add_master=False)
-                except Exception, err:
-                    print err
+                self.append(l, add_master=False)
         else:
             if force_append:
                 self.config._valid_len(self.opt._name, lst)
@@ -84,7 +81,8 @@ class Multi(list):
         except TypeError:
             self._setvalue(value, who=settings.get_owner())
         multis = []
-        for name, multi in self.config:
+        for opt in self.config._cfgimpl_descr._children:
+            multi = self.config._cfgimpl_values[opt._name]
             multis.append(multi)
         for multi in multis:
             if master == multi.opt._name: