Make use of python-subprocess32 module if available.

python-subprocess32 [https://code.google.com/p/python-subprocess32/]
is a backport of Python 3.2 subprocess module with:

* Better process forking.
* More reliable when running with threads.

Although we're not going to use any feature of subprocess32,
this change turn it possible to use subprocess32 as a drop-in
replacement for the original subprocess module.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #285 from ruda/optional_subprocess32

Make use of python-subprocess32 module if available.