Expand sudo_user after variable merging

Previous commit c3659741 expanded sudo_user during task construction,
but this is too early as it does not pick up variables set during
the play.

This commit moves sudo_user expansion to the runner after variables
have been merged.

lib/ansible/playbook/task.py

@@ -115,7 +115,7 @@ class Task(object):
         self.args         = ds.get('args', {})
 
         if self.sudo:
-            self.sudo_user    = template.template(play.basedir, ds.get('sudo_user', play.sudo_user), module_vars)
+            self.sudo_user    = ds.get('sudo_user', play.sudo_user)
             self.sudo_pass    = ds.get('sudo_pass', play.playbook.sudo_pass)
         else:
             self.sudo_user    = None