Adding warning messages to insecure LDAP configurations and added --ldap_passwords_in_clear_ok

Change-Id: Id7c7006269c11b4cd7aea51789b7af9aeffea2c3
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3501
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: Henry Robinson <henry@cloudera.com>