Address ASF feedback for RC3

[Gerrrr]

• The NOTICE file does not include '(Incubating)' after the podling name.
  https://github.com/apache/amoro/blob/master/NOTICE is a valid example
• No DISCLAIMER in otava-0.6.0+incubating.tar.gz or in
  otava-0.6.0+incubating-py3-none-any.whl
• The email address in otava-0.6.0+incubating.dist-info/METADATA (in
  the wheel file) has an individual's non Apache email address
  I think this should have an address like dev@otava.apache.org. It is
  not the ASF norm for individual's to take the credit for maintaining
  files - it is a team effort.

I would prefer if the LICENSE, NOTICE and DISCLAIMER had no .txt
suffix. This is not the norm but probably allowable.

See individual commits for each point.