Restrict maximum attribute size from formatting

If an expression causes an inordinate amount of memory to be
used, then block it from continuing.

For now, we consider that an expression that expands beyond 16k.  I
am unable to conceive of a use case where someone would want to
use an expression to derive more than 16k as it stands, as we don't
carry any particularly large opaque payloads right now.

confluent_server/confluent/config/configmanager.py

@@ -1112,7 +1112,10 @@ class _ExpressionFormat(string.Formatter):
             val = int(val)
         except Exception:
             pass
-        return format(val, format_spec)
+        formatted = format(val, format_spec)
+        if len(formatted) > 16384:
+            raise Exception('Field length exceeded during formatting')
+        return formatted
 
     def _handle_ast_node(self, node):
         if isinstance(node, ast.Num):