Validate top level of the layout configuration, too

I got an exception that a list object has no method "get" when I
accidentally put the contents of my pipelines as the top level object in
the layout YAML file.

Change-Id: Ia88f2a849bbc1dca5ec44afc969ddfa9c3598f1e

zuul/layoutvalidator.py

@@ -192,6 +192,9 @@ class LayoutSchema(object):
         return parameters
 
     def getSchema(self, data):
+        if not isinstance(data, dict):
+            raise Exception("Malformed layout configuration: top-level type "
+                            "should be a dictionary")
         pipelines = data.get('pipelines')
         if not pipelines:
             pipelines = []