luci-app-openvpn: more changes & fixes

* fix possible exception in template based ovpn creation
* remove needless shellquote function in controller,
  the filename will be checked on client side with JS
* enhance FileUpload behaviour in basic/advanced mode:
  - change "auth_user_pass" to FileUpload
  - cfg entries (even with default values) will be shown
  - existing entries are now removable (incl. file unlink),
    simply clear the appropriate textbox
* change "key_direction" option to boolean ListValue
* add "config" option to basic/advanced edit,
  to make it possible to change the upload path in LuCI

Signed-off-by: Dirk Brenken <dev@brenken.org>