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). Note that subprocess32 was
already installed because it's an indirect dependency
(cliff->cmd2->subprocess32),  make it a direct dependency now that we
are using it directly.

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