fixing problem with reindex-fast on CKAN 2.7

I didn't test this on 2.8 but it might be needed there as well from what i see in the code there

ckan/lib/cli.py

@@ -627,7 +627,7 @@ def clear(self):
 
     def rebuild_fast(self):
         ###  Get out config but without starting pylons environment ####
-        conf = self._get_config()
+        conf = _get_config(self.options.config)
 
         ### Get ids using own engine, otherwise multiprocess will balk
         db_url = conf['sqlalchemy.url']