Fix sphinx docs build

When building docs with sphinx, the objects.register_all() code path is
not necessarily called, and therefore objects.* is not necessarily
imported before it is used. This throws an error while building the
sample ChassisCollection response.

If objects.Chassis is not available, call register_all() before using
it.

Change-Id: Ic0041085639f3d5a900ad54c2637c77aa467ba35
Closes-Bug: #1560508

ironic/api/controllers/v1/chassis.py

@@ -143,6 +143,9 @@ class ChassisCollection(collection.Collection):
 
     @classmethod
     def sample(cls):
+        # FIXME(jroll) hack for docs build, bug #1560508
+        if not hasattr(objects, 'Chassis'):
+            objects.register_all()
         sample = cls()
         sample.chassis = [Chassis.sample(expand=False)]
         return sample