tests/slapproxy: discard child slapos proxy output

to keep tests silent.

On python2, this uses subprocess32, which backports DEVNULL (and
default setting of closing file descriptors). This also adds a
dependency to subproces32 on python 2.7

setup.py

@@ -54,6 +54,7 @@ setup(name=name,
           'zope.interface', # slap library implementes interfaces
           'zc.buildout',
           'cliff',
+          'subprocess32; python_version<"3"',
           'requests>=2.4.3',
           'six',
           'uritemplate', # used by hateoas navigator

slapos/tests/slapproxy/__init__.py

@@ -34,7 +34,10 @@ import os
 import logging
 import shutil
 import socket
-import subprocess
+try:
+  import subprocess32 as subprocess
+except ImportError:
+  import subprocess
 import sys
 import tempfile
 import time
@@ -1029,6 +1032,8 @@ database_uri = %(tempdir)s/lib/external_proxy.db
             sys.executable, '%s/../cli/entry.py' % os.path.dirname(slapos.tests.__file__),
             'proxy', 'start', '--cfg', self.external_slapproxy_configuration_file_location
         ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
         env={"PYTHONPATH": ':'.join(sys.path)}
     )
     # Wait a bit for proxy to be started