make check output style consistent

The RAT and PEP8 checks don’t print a blank line after successful runs.
The scalastyle check shouldn’t either.
[SPARK-2627] add Python linter

This guy just runs the pep8 utility on all code in the python
directory, minus cloudpickle, which is a 3rd-party library.
[SPARK-2627] add Scala linter

This guy just calls scalastyle.
[SPARK-2627] call Python linter from run-tests
always delete the report file