Fix EZP-20518: Default layout for legacy content fallback configurable by SA

Jira issue: https://jira.ez.no/browse/EZP-20518

# Description
This patch adds the ability to define by siteaccess the pagelayout to use while rendering the content in the legacy

See also https://github.com/ezsystems/ezpublish-community/pull/44

# Test
manual tests