Fix with using oslo_serialization base64 module

The test_can_create_server_with_max_number_personality_files
Tempest test is expected to use base64 module from oslo
serialization package, but the b64decode() from std Python lib
is called. The current patch fixes it.

Change-Id: Icf23aa128b8f857f98096e86ff945241efa96b9a
Closes-Bug: #1625079
diff --git a/tempest/api/compute/servers/test_server_personality.py b/tempest/api/compute/servers/test_server_personality.py
index cd90473..e5ad7b4 100644
--- a/tempest/api/compute/servers/test_server_personality.py
+++ b/tempest/api/compute/servers/test_server_personality.py
@@ -137,6 +137,6 @@
                 server=server,
                 servers_client=self.client)
             for i in person:
-                self.assertEqual(base64.b64decode(i['contents']),
+                self.assertEqual(base64.decode_as_text(i['contents']),
                                  linux_client.exec_command(
                                      'sudo cat %s' % i['path']))