Fix sockaddr lib with python 3

Padding did not work and this lib did not have tests

Sample failure seen in neutron-dynamic-routing:
neutron-bgp-dragent[29325]: TypeError: can't concat str to bytes

Story: 2007910
Task: 40311
Change-Id: If616980b2c6d7303cdc5d4f5e7247d63e4c34939

os_ken/lib/sockaddr.py

@@ -54,7 +54,7 @@ def _hdr(ss_len, af):
 
 def _pad_to(data, total_len):
     pad_len = total_len - len(data)
-    return data + pad_len * '\0'
+    return data + pad_len * b'\0'
 
 
 def sa_in4(addr, port=0):

os_ken/tests/unit/lib/test_sockaddr.py

@@ -0,0 +1,54 @@
+# Copyright 2020 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import unittest
+import logging
+import platform
+from nose.tools import eq_
+
+from os_ken.lib import sockaddr
+
+system = platform.system()
+
+
+class Test_sockaddr(unittest.TestCase):
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def test_sockaddr_linux_sa_in4(self):
+        if system != 'Linux':
+            return
+
+        addr = '127.0.0.1'
+        expected_result = (b'\x02\x00\x00\x00'
+            b'\x7f\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00')
+        eq_(expected_result, sockaddr.sa_in4(addr))
+
+    def test_sockaddr_linux_sa_in6(self):
+        if system != 'Linux':
+            return
+
+        addr = 'dead:beef::1'
+        expected_result = (b'\n\x00\x00\x00\x00\x00\x00\x00\xde\xad\xbe\xef'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00')
+        eq_(expected_result, sockaddr.sa_in6(addr))
+
+    def test_sockaddr_sa_to_ss(self):
+        addr = b'\x01'
+        expected_result = b'\x01' + 127 * b'\x00'
+        eq_(expected_result, sockaddr.sa_to_ss(addr))