Correcly parse Python traceback

Change-Id: Ic937623e5d03dcbf8d7510d4d91311b9a4c06521
diff --git a/heka/files/lua/decoders/openstack_log.lua b/heka/files/lua/decoders/openstack_log.lua
index 9199bd4..95b2002 100644
--- a/heka/files/lua/decoders/openstack_log.lua
+++ b/heka/files/lua/decoders/openstack_log.lua
@@ -101,7 +101,7 @@
         end
     end
 
-    if patt.traceback:match(m.Message) then
+    if patt.anywhere(patt.traceback):match(m.Message) then
         -- Python traceback detected, begin accumulating the lines making
         -- up the traceback.
         traceback_key = key