Apache Yetus integration

The plugin under buildtest.sh will provide the plugin to run the base build.sh test which will trigger the tests of the main Avro projects, and the sections below lang/*/build.sh. To run this, use:

test-patch --plugins=buildtest --user-plugins=share/precommit/ --run-tests --empty-patch --docker --dockerfile=share/docker/Dockerfile --dirty-workspace --verbose=true

Note, that this is still a very crude implementation of Apache Yetus, and in the future we would like to refine this to provide plugins for the different languages.