[SPARK-2627] download pep8 at runtime

See the discussion here:
#1744 (comment)

Get the pep8 utility at runtime so that it’s not required to be
installed on the build server.

dev/lint-python

@@ -22,11 +22,21 @@ SPARK_ROOT_DIR="$(dirname $SCRIPT_DIR)"
 PEP8_REPORT_PATH="$SPARK_ROOT_DIR/dev/pep8-report.txt"
 
 cd $SPARK_ROOT_DIR
+
+# See: https://github.com/apache/spark/pull/1744#issuecomment-50982162
+# Get pep8 at runtime so that we don't rely on it being installed on the build server.
+# TODOs:
+#  - Dynamically determine latest release version of pep8 and use that.
+#  - Download this from a more reliable source. (GitHub raw can be flaky, apparently. (?))
+PEP8_SCRIPT_PATH="$SPARK_ROOT_DIR/dev/pep8.py"
+curl --silent -o "$PEP8_SCRIPT_PATH" \
+    "https://raw.githubusercontent.com/jcrocholl/pep8/1.5.7/pep8.py"
+
 # There is no need to write this output to a file
 #+ first, but we do so so that the check status can
 #+ be output before the report, like with the
 #+ scalastyle and RAT checks.
-pep8 ./python --exclude="cloudpickle.py" \
+python $PEP8_SCRIPT_PATH ./python --exclude="cloudpickle.py" \
     > "$PEP8_REPORT_PATH"
 pep8_status=${PIPESTATUS[0]} #$?
 
@@ -39,4 +49,6 @@ if [ $pep8_status -ne 0 ]
 fi
 
 rm -f "$PEP8_REPORT_PATH"
+rm "$PEP8_SCRIPT_PATH"
+
 exit $pep8_status