Fix EZP-22839: DemoBundle settings hardcoded to specific siteaccess names/group

> https://jira.ez.no/browse/EZP-22839

Changed `ezdemo_frontend_group` to `default`.
This was not possible before since `default` keyword wasn't taken into
account in semantic config.

Also added pagelayout configuration for legacy to its own file, using
semantic configuration for EzLegacyBundle. Thus
`ezpublish.default.view_default_layout` should be removed from default
`parameters.yml.dist`.

(cherry picked from commit d5ea68c94c5a179d8698dfed3af04a4a989f358f)