Backward compatiblity about getInstanceGuid().

(cherry picked from commit ae9f790de75373fbe2d18e2cbd28a1a02c62ad71)

slapos/recipe/request.py

@@ -131,8 +131,8 @@ class Recipe(object):
           options['instance-guid'] = self.instance.getInstanceGuid()
           # XXX: deprecated, to be removed
           options['instance_guid'] = self.instance.getInstanceGuid()
-        except slapmodule.ResourceNotReady:
-          # Backward compatibility. Old SlapOS masters don't know this.
+        except slapmodule.ResourceNotReady, AttributeError:
+          # Backward compatibility. Old SlapOS master and core don't know this.
           self.logger.warning("Impossible to fetch instance GUID.")
     except (slapmodule.NotFoundError, slapmodule.ServerError, slapmodule.ResourceNotReady) as exc:
       self._raise_request_exception = exc