Allow to disable the default ezdemo_frontend_group siteaccess group

DemoBundle prepends the ezdemo_frontend_group siteaccess group configuration.

This can become an issue when extending DemoBundle as it must be loaded first.  This makes ezdemo_frontend_group take precedence over any other siteaccess group a user may define and does not allow the user to define the order on their own.

This allows a user to define ezdemo.ezdemo_frontend_group.siteaccess_group_enabled: false in ezpublish.yml to disable this behaviour and allow the user to define the group order they wish.