Fix the argument of UnknownArgument exception

This change sets the real UnknownArgument instead of the last
valid one in the exception raised by the API.

Fixes bug #1201839

Change-Id: I22e1dadbe9a90e03e8ccfbcc6e0e8dc3f17c5a3e

ceilometer/api/controllers/v2.py

@@ -213,8 +213,7 @@ def _query_to_kwargs(query, db_func):
     if trans:
         for k in trans:
             if k not in valid_keys:
-                raise wsme.exc.UnknownArgument(i.field,
-                                               "unrecognized query field")
+                raise wsme.exc.UnknownArgument(k, "unrecognized query field")
             kwargs[k] = trans[k]
 
     return kwargs