Handle compute node records with no timestamp.

Commit 94560ab added a fast-path
to avoid work when baremetal compute nodes haven't changed, but if
the compute message has None for its updated-at timestamp (which
can happen during startup) the comparison crashes.

Fixes bug: 1096719
Change-Id: Ib8add431d975eee133deefea62eb53479a97396e
(cherry picked from commit fad69df)

nova/scheduler/host_manager.py

@@ -122,7 +122,8 @@ def update_capabilities(self, topic, capabilities=None, service=None):
 
     def update_from_compute_node(self, compute):
         """Update information about a host from its compute_node info."""
-        if self.updated and self.updated > compute['updated_at']:
+        if (self.updated and compute['updated_at']
+            and self.updated > compute['updated_at']):
             return
         all_ram_mb = compute['memory_mb']