[MINOR][PYTHON] Ignore pep8 on test scripts generated in tests in work directory

## What changes were proposed in this pull request?

Currently, if we run `./python/run-tests.py` and they are aborted without cleaning up this directory, it fails pep8 check due to some Python scripts generated. For example, https://github.com/apache/spark/blob/7387126f83dc0489eb1df734bfeba705709b7861/python/pyspark/tests.py#L1955-L1968

```
PEP8 checks failed.
./work/app-20170531190857-0000/0/test.py:5:55: W292 no newline at end of file
./work/app-20170531190909-0000/0/test.py:5:55: W292 no newline at end of file
./work/app-20170531190924-0000/0/test.py:3:1: E302 expected 2 blank lines, found 1
./work/app-20170531190924-0000/0/test.py:7:52: W292 no newline at end of file
./work/app-20170531191016-0000/0/test.py:5:55: W292 no newline at end of file
./work/app-20170531191030-0000/0/test.py:5:55: W292 no newline at end of file
./work/app-20170531191045-0000/0/test.py:3:1: E302 expected 2 blank lines, found 1
./work/app-20170531191045-0000/0/test.py:7:52: W292 no newline at end of file
```

For me, it is sometimes a bit annoying. This PR proposes to exclude these (assuming we want to skip per https://github.com/apache/spark/blob/master/.gitignore#L73).

Also, it moves other pep8 configurations in the script into ini configuration file in pep8.

## How was this patch tested?

Manually tested via `./dev/lint-python`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18161 from HyukjinKwon/work-exclude-pep8.

dev/lint-python

@@ -20,7 +20,7 @@
 SCRIPT_DIR="$( cd "$( dirname "$0" )" && pwd )"
 SPARK_ROOT_DIR="$(dirname "$SCRIPT_DIR")"
 # Exclude auto-geneated configuration file.
-PATHS_TO_CHECK="$( cd "$SPARK_ROOT_DIR" && find . -name "*.py" -not -path "*python/docs/conf.py" )"
+PATHS_TO_CHECK="$( cd "$SPARK_ROOT_DIR" && find . -name "*.py" )"
 PEP8_REPORT_PATH="$SPARK_ROOT_DIR/dev/pep8-report.txt"
 PYLINT_REPORT_PATH="$SPARK_ROOT_DIR/dev/pylint-report.txt"
 PYLINT_INSTALL_INFO="$SPARK_ROOT_DIR/dev/pylint-info.txt"
@@ -64,7 +64,7 @@ export "PATH=$PYTHONPATH:$PATH"
 #+ first, but we do so so that the check status can
 #+ be output before the report, like with the
 #+ scalastyle and RAT checks.
-python "$PEP8_SCRIPT_PATH" --ignore=E402,E731,E241,W503,E226 --config=dev/tox.ini $PATHS_TO_CHECK >> "$PEP8_REPORT_PATH"
+python "$PEP8_SCRIPT_PATH" --config=dev/tox.ini $PATHS_TO_CHECK >> "$PEP8_REPORT_PATH"
 pep8_status="${PIPESTATUS[0]}"
 
 if [ "$compile_status" -eq 0 -a "$pep8_status" -eq 0 ]; then

dev/tox.ini

@@ -14,5 +14,6 @@
 # limitations under the License.
 
 [pep8]
+ignore=E402,E731,E241,W503,E226
 max-line-length=100
-exclude=cloudpickle.py,heapq3.py,shared.py
+exclude=cloudpickle.py,heapq3.py,shared.py,python/docs/conf.py,work/*/*.py