Fixing a few bugs in the HostVars performance areas

* Also refresh inventory in the HostVars manager process when things
  are changed via add_host/group_by
* Raise j2undefined rather than return it

lib/ansible/vars/__init__.py

@@ -119,7 +119,7 @@ class VariableManager:
         self._host_vars_files = data.get('host_vars_files', defaultdict(dict))
         self._group_vars_files = data.get('group_vars_files', defaultdict(dict))
         self._omit_token = data.get('omit_token', '__omit_place_holder__%s' % sha1(os.urandom(64)).hexdigest())
-        self._inventory = None
+        self._inventory = data.get('inventory', None)
 
     def _get_cache_entry(self, play=None, host=None, task=None):
         play_id = "NONE"

lib/ansible/vars/hostvars.py

@@ -57,13 +57,16 @@ class HostVars(collections.Mapping):
     def set_variable_manager(self, variable_manager):
         self._variable_manager = variable_manager
 
+    def set_inventory(self, inventory):
+        self._inventory = inventory
+
     def _find_host(self, host_name):
         return self._inventory.get_host(host_name)
 
     def __getitem__(self, host_name):
         host = self._find_host(host_name)
         if host is None:
-            return j2undefined
+            raise j2undefined
 
         data = self._variable_manager.get_vars(loader=self._loader, host=host, play=self._play, include_hostvars=False)