Set ansible stdin to /dev/null

Without this, when using the nullwrap driver, ansible throws the
following exception:

  AttributeError: 'NoneType' object has no attribute 'isatty'

Change-Id: Ic2d769fa1a0a1faf173aa36d33b291065b44a18b

zuul/executor/server.py

@@ -1300,6 +1300,7 @@ class AnsibleJob(object):
             self.proc = popen(
                 cmd,
                 cwd=self.jobdir.work_root,
+                stdin=subprocess.DEVNULL,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT,
                 preexec_fn=os.setsid,