Avoid importing Linux specific modules on Windows

oslo.rootwrap currently imports a few Linux-specific modules, which
means that any services importing oslo.rootwrap modules will fail to
start on Windows (e.g.: neutron-ovs-agent).

Change-Id: I229c311939facdffdcde484f590e980102024b5a
Closes-Bug: #1661326

oslo_rootwrap/filters.py

@@ -14,9 +14,14 @@
 #    under the License.
 
 import os
-import pwd
 import re
 import shutil
+import sys
+
+if sys.platform != 'win32':
+    # NOTE(claudiub): pwd is a Linux-specific library, and currently there is
+    # no Windows support for oslo.rootwrap.
+    import pwd
 
 
 def _getuid(user):

oslo_rootwrap/wrapper.py

@@ -16,8 +16,11 @@
 import logging
 import logging.handlers
 import os
-import pwd
 import signal
+import sys
+
+if sys.platform != 'win32':
+    import pwd
 
 import six
 from six import moves