Add InvalidCredentials to tempest-lib

This exception is used in tempest-lib and should have been
migrated as part of auth.py. Adding it now.

Change-Id: I4eea69260a7492e5d1a2ab3a7418b7d8bbea81db

tempest_lib/exceptions.py

@@ -151,3 +151,7 @@ class IdentityError(TempestException):
 
 class EndpointNotFound(TempestException):
     message = "Endpoint not found"
+
+
+class InvalidCredentials(TempestException):
+    message = "Invalid Credentials"