Fix --force-handlers, and allow it in plays and ansible.cfg

The --force-handlers command line argument was not correctly running
handlers on hosts which had tasks that later failed. This corrects that,
and also allows you to specify force_handlers in ansible.cfg or in a
play.

test/units/TestPlayVarsFiles.py

@@ -47,6 +47,7 @@ class FakePlayBook(object):
         self.transport = None
         self.only_tags = None
         self.skip_tags = None
+        self.force_handlers = None
         self.VARS_CACHE = {}
         self.SETUP_CACHE = {}
         self.inventory = FakeInventory()