GetAtt can return objects as well (#2709)

Author: kddejong

src/cfnlint/decode/node.py

@@ -96,7 +96,7 @@ def is_function_returning_object(self, mappings=None):
                                 if isinstance(p_v, dict):
                                     if len(p_v) == 1:
                                         for l_k in p_v.keys():
-                                            if l_k == "Fn::FindInMap":
+                                            if l_k in ["Fn::FindInMap", "Fn::GetAtt"]:
                                                 return True
 
             return False