INNODB_UNDO_TABLESPACES=2

  - innodb_undo_directory had a default value as "." which is represent
    current directory.
  - In embedded mode data-directory is not same as current directory
    of the program and so during embedded mode server may not be able to
    locate undo-tablespace created during initialization.

  Fix:
  - Changed default value of innodb_undo_directory to NULL.
  - If it is not set it will be set to default_dir which is data-dir.
  - If it is is set explictly then settings are honored.

  Behavior is now consistent with other path too.

  Reviewed by: Kevin Lewis (kevin.lewis@oracle.com)
  RB: 8777