[SPARK-8763] [PYSPARK] executing run-tests.py with Python 2.6 fails with...

[SPARK-8763] [PYSPARK] executing run-tests.py with Python 2.6 fails with absence of subprocess.check_output function

Running run-tests.py with Python 2.6 cause following error:

```
Running PySpark tests. Output is in python//Users/tomohiko/.jenkins/jobs/pyspark_test/workspace/python/unit-tests.log
Will test against the following Python executables: ['python2.6', 'python3.4', 'pypy']
Will test the following Python modules: ['pyspark-core', 'pyspark-ml', 'pyspark-mllib', 'pyspark-sql', 'pyspark-streaming']
Traceback (most recent call last):
  File "./python/run-tests.py", line 196, in <module>
    main()
  File "./python/run-tests.py", line 159, in main
    python_implementation = subprocess.check_output(
AttributeError: 'module' object has no attribute 'check_output'
...
```

The cause of this error is using subprocess.check_output function, which exists since Python 2.7.
(ref. https://docs.python.org/2.7/library/subprocess.html#subprocess.check_output)

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #7161 from cocoatomo/issues/8763-test-fails-py26 and squashes the following commits:

cf4f901 [cocoatomo] [SPARK-8763] backport process.check_output function from Python 2.7

python/run-tests.py

@@ -31,6 +31,23 @@ if sys.version < '3':
     import Queue
 else:
     import queue as Queue
+if sys.version_info >= (2, 7):
+    subprocess_check_output = subprocess.check_output
+else:
+    # SPARK-8763
+    # backported from subprocess module in Python 2.7
+    def subprocess_check_output(*popenargs, **kwargs):
+        if 'stdout' in kwargs:
+            raise ValueError('stdout argument not allowed, it will be overridden.')
+        process = subprocess.Popen(stdout=subprocess.PIPE, *popenargs, **kwargs)
+        output, unused_err = process.communicate()
+        retcode = process.poll()
+        if retcode:
+            cmd = kwargs.get("args")
+            if cmd is None:
+                cmd = popenargs[0]
+            raise subprocess.CalledProcessError(retcode, cmd, output=output)
+        return output
 
 
 # Append `SPARK_HOME/dev` to the Python path so that we can import the sparktestsupport module
@@ -156,11 +173,11 @@ def main():
 
     task_queue = Queue.Queue()
     for python_exec in python_execs:
-        python_implementation = subprocess.check_output(
+        python_implementation = subprocess_check_output(
             [python_exec, "-c", "import platform; print(platform.python_implementation())"],
             universal_newlines=True).strip()
         LOGGER.debug("%s python_implementation is %s", python_exec, python_implementation)
-        LOGGER.debug("%s version is: %s", python_exec, subprocess.check_output(
+        LOGGER.debug("%s version is: %s", python_exec, subprocess_check_output(
             [python_exec, "--version"], stderr=subprocess.STDOUT, universal_newlines=True).strip())
         for module in modules_to_test:
             if python_implementation not in module.blacklisted_python_implementations: