Update spring-xd-yarn xd-config.yml

- Update batch initializer config in spring-xd-yarn's xd-config.yml
   - Have it commented out by default
Enable parallel tests

- Modify RandomConfigurationSupport's deleteFiles to print stacktrace in case of failure while
deleting
- Increase `forkEvery` to 20 in case of spring-xd-shell
- Running -PmaxParallelForks=<?> improve build time depending on the dev machine configuration