Add incidents_dir config field

On the [node] section of the tahoe.cfg now there is a field 'incidents_dir'
where the path to the incidents foder can be configured.
return default introducer web.port to none

No web ui for introducers has been the default behavior for a long time, so
lets leave it that way now even though create-introducer can configure a
web.port now. This also makes the one test that was failing pass.