Change hostvars to a mutable mapping to more closely resemble a dict

Fixes #4030

lib/ansible/runner/__init__.py

@@ -81,13 +81,20 @@ def _executor_hook(job_queue, result_queue, new_stdin):
         except:
             traceback.print_exc()
 
-class HostVars(collections.Mapping):
+class HostVars(collections.MutableMapping):
     ''' A special view of setup_cache that adds values from the inventory when needed. '''
 
     def __init__(self, setup_cache, inventory):
         self.setup_cache = setup_cache
         self.inventory = inventory
-        self.lookup = {}
+        self.lookup = dict()
+        self.update(setup_cache)
+
+    def __setitem__(self, host, value):
+        self.lookup[host] = value
+
+    def __delitem__(self, host):
+        del self.lookup[host]
 
     def __getitem__(self, host):
         if not host in self.lookup:
@@ -97,10 +104,10 @@ class HostVars(collections.Mapping):
         return self.lookup[host]
 
     def __iter__(self):
-        return (host.name for host in self.inventory.get_group('all').hosts)
+        return iter(self.lookup)
 
     def __len__(self):
-        return len(self.inventory.get_group('all').hosts)
+        return len(self.lookup)
 
 
 class Runner(object):