SPARK-1990: added compatibility for python 2.6 for ssh_read command

https://issues.apache.org/jira/browse/SPARK-1990

There were some posts on the lists that spark-ec2 does not work with Python 2.6. In addition, we should check the Python version at the top of the script and exit if it's too old

Author: Anant <anant.asty@gmail.com>

Closes #941 from anantasty/SPARK-1990 and squashes the following commits:

4ca441d [Anant] Implmented check_optput withinthe module to work with python 2.6
c6ed85c [Anant] added compatibility for python 2.6 for ssh_read command

ec2/spark_ec2.py

@@ -689,9 +689,23 @@ def ssh(host, opts, command):
             time.sleep(30)
             tries = tries + 1
 
+# Backported from Python 2.7 for compatiblity with 2.6 (See SPARK-1990)
+def _check_output(*popenargs, **kwargs):
+    if 'stdout' in kwargs:
+        raise ValueError('stdout argument not allowed, it will be overridden.')
+    process = subprocess.Popen(stdout=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
+
 
 def ssh_read(host, opts, command):
-    return subprocess.check_output(
+    return _check_output(
         ssh_command(opts) + ['%s@%s' % (opts.user, host), stringify_command(command)])