feat #797 Improvement of page definition for page engine

Two features are added with this commit:

1 - property "contents" is no longer mandatory in a page composition
2 - The bean definition for "placeholderContents" has been changed in order to reflect the code: values in the map can be either a single content or an array of them. The code was already taking that possibility into account. Tests have been added also for the case in which a template and a contentId are set in a placeholder configuration.

Close #797