Don't capture stderr in siblings logic

When python setup.py --name is run in ansible/ansible currently, a
warning is printed:

  UserWarning: Normalizing '2.7.0dev0' to '2.7.0.dev0' normalized_version

With stderr=stdout, this goes into the package name causing sibling
detection to not work.

The stderr is not interesting to us from this command, so ignore it.

Change-Id: I38ea21960932dbddcac1b828e95a0cf963c3b94c

roles/tox/library/tox_install_sibling_packages.py

@@ -93,7 +93,7 @@ def get_sibling_python_packages(projects, tox_python):
             # package name is.
             package_name = subprocess.check_output(
                 [os.path.abspath(tox_python), 'setup.py', '--name'],
-                cwd=os.path.abspath(root), stderr=subprocess.STDOUT)
+                cwd=os.path.abspath(root))
             if package_name:
                 package_name = package_name.strip()
                 packages[package_name] = root