Update overcloud examples for ExtraConfig

The existing examples for the overcloud ExtraConfig options
use an ironic setting that would likely never apply (Ironic
isn't for the overcloud).

This patch modifies the default section setting to use
the Nova force_config_drive option instead as a
config example.

Change-Id: Ieb893552fe9466b90b9d9a831a676d114efb6db1
Merge "Update overcloud examples for ExtraConfig"