Fix Unexpected Exception: global name 'os' is not defined

I inadvertently introduced it in
ca826508 and didn't notice, because
there are no unit tests for playbook_executor.py.  Sorry!

(The "from ansible.errors import *" was used *only* to get the 'os'
module, which makes go "what?")

lib/ansible/executor/playbook_executor.py

@@ -21,13 +21,13 @@ __metaclass__ = type
 
 import getpass
 import locale
+import os
 import signal
 import sys
 
 from six import string_types
 
 from ansible import constants as C
-from ansible.errors import *
 from ansible.executor.task_queue_manager import TaskQueueManager
 from ansible.playbook import Playbook
 from ansible.template import Templar