Change bytes to str in servers_client for python3

In python3, the response body of clients will be of type bytes,
but jsonschema.validate expects parameter of type str, so we need
change bytes to str when needed, otherwise we will get errors like
"HTTP response body is invalid (b'' is not of type 'object', 'string'"

Change-Id: If4cad1b0f706aeb5f83931cb6928806a2c5cee8e
Closes-Bug: #1864625

tempest/lib/services/compute/servers_client.py

@@ -209,6 +209,9 @@ class ServersClient(base_compute_client.BaseComputeClient):
                                post_body)
         if body:
             body = json.loads(body)
+        else:
+            if isinstance(body, bytes):
+                body = body.decode('utf-8')
         self.validate_response(schema, resp, body)
         return rest_client.ResponseBody(resp, body)