some fixes to become/sudo

* now it uses -n to get immediate error if no password is supplied and one is needed,
   this should fix the issue with sudo hanging waiting for input.
 * made -k configurable, this can break changing become_users in play if left out,
   but opens up the possiblity of OTP support.

examples/ansible.cfg

@@ -46,8 +46,9 @@ gathering = implicit
 # change this for alternative sudo implementations
 sudo_exe = sudo
 
-# what flags to pass to sudo
-#sudo_flags = -H
+# What flags to pass to sudo
+# WARNING: leaving out the defaults might create unexpected behaviours
+#sudo_flags = -H -k
 
 # SSH timeout
 timeout = 10