Convert str to bytes on wsproxy

In python3, concatenating string to bytes is not supported so
zun-wsproxy broke. This commit convert string to bytes before
sending the buffer.

Change-Id: I236d5c39b0415cc345a11ab5e79d7e7082f367c9
Closes-Bug: #1854090

zun/websocket/websocketproxy.py

@@ -26,6 +26,7 @@ import time
 import docker
 from oslo_log import log as logging
 from oslo_utils import uuidutils
+import six
 import six.moves.urllib.parse as urlparse
 import websockify
 
@@ -120,6 +121,8 @@ class ZunProxyRequestHandlerBase(object):
                     'host': self.server.target_host,
                     'port': self.server.target_port})
                 raise self.CClose(1000, "Target closed")
+            if isinstance(buf, six.string_types):
+                buf = buf.encode()
             self.cqueue.append(buf)
 
     def do_websocket_proxy(self, target):