Fix missing value types for log message

This commit fix the missing value type in log message of exception.

Change-Id: Ib052964e49ded2eb427d5b448241e25cbd066906
Closes-Bug: #1494574
diff --git a/tempest/exceptions.py b/tempest/exceptions.py
index 15482ab..b3d60f6 100644
--- a/tempest/exceptions.py
+++ b/tempest/exceptions.py
@@ -64,7 +64,7 @@
 
 
 class InvalidIdentityVersion(TempestException):
-    message = "Invalid version %(identity_version) of the identity service"
+    message = "Invalid version %(identity_version)s of the identity service"
 
 
 class TimeoutException(TempestException):