Fix table rendering for iterable objects

Table rendering is broken for objects which are iterable but do NOT
implement a 'get' method. This patch ensures that we don't try to
invoke a 'get' method on objects which don't support it.

Change-Id: If29946f8c553b85a03ea6c50b0cf74f3ba787d0b
Closes-Bug: #1278956

horizon/tables/base.py

@@ -307,7 +307,7 @@ class Column(html.HTMLElement):
         if callable(self.transform):
             data = self.transform(datum)
         # Dict lookups
-        elif isinstance(datum, collections.Iterable) and \
+        elif isinstance(datum, collections.Mapping) and \
                 self.transform in datum:
             data = datum.get(self.transform)
         else: