stack: add a version pin for subprocess32 ( 3.5.3 )

It will be used on python2.7

from https://docs.python.org/2/library/subprocess.html :
POSIX users (Linux, BSD, etc.) are strongly encouraged to install and
use the much more recent subprocess32 module instead of the version
included with python 2.7. It is a drop in replacement with better
behavior in many situations.

/reviewed-on !476

stack/slapos.cfg

@@ -144,6 +144,7 @@ slapos.recipe.build = 0.40
 slapos.recipe.cmmi = 0.10
 slapos.toolbox = 0.84
 stevedore = 1.21.0
+subprocess32 = 3.5.3
 unicodecsv = 0.14.1
 xml-marshaller = 0.9.7
 paramiko = 2.1.3