Adds methods for [] & len into LazyURLPattern

The Django 404 page currently depends on being able to call len()
and index LazyURLPattern.  However, the special methods for doing
this (__len__() and __getitem__()) are not forwarded to the object
wrapped by SimpleLazyObject by default.  Thus, methods to foward
these special methods to the wrapped object have been implemented
in LazyURLPattern.

Fixes: bug 1183578
Change-Id: I133e97ed66180fe78cfe1b7fd9c86bac4171a994

horizon/base.py

@@ -561,6 +561,16 @@ class LazyURLPattern(SimpleLazyObject):
             self._setup()
         return reversed(self._wrapped)
 
+    def __len__(self):
+        if self._wrapped is empty:
+            self._setup()
+        return len(self._wrapped)
+
+    def __getitem__(self, idx):
+        if self._wrapped is empty:
+            self._setup()
+        return self._wrapped[idx]
+
 
 class Site(Registry, HorizonComponent):
     """ The overarching class which encompasses all dashboards and panels. """