Init HostState.failed_builds

If _update_from_compute_node returns early and the HostState is
not filtered out we can hit an AttributeError in the
BuildFailureWeigher because the failed_builds attribute is not
set. This simply initializes the attribute like the other stats
fields.

Change-Id: I5f8e4d32c6a1d6b61396b4fa11c5d776f432df0c
Closes-Bug: #1834691

Author: mriedem

nova/scheduler/host_manager.py

@@ -126,6 +126,7 @@ def __init__(self, host, node, cell_uuid):
         # Additional host information from the compute node stats:
         self.num_instances = 0
         self.num_io_ops = 0
+        self.failed_builds = 0
 
         # Other information
         self.host_ip = None

nova/tests/unit/scheduler/test_host_manager.py

@@ -1573,3 +1573,5 @@ def test_stat_consumption_from_compute_node_not_ready(self):
         # Because compute record not ready, the update of free ram
         # will not happen and the value will still be 0
         self.assertEqual(0, host.free_ram_mb)
+        # same with failed_builds
+        self.assertEqual(0, host.failed_builds)