Fix query to get Template documents

The query used by TemplateMappingFetcher is

 select * from Document where tmpl:forcedTypes/* <> 'none'

With VCS, this query only returns documents which contain the documenttemplate schema, so using from Document does not have an impact.

With MongoDB, this query returns all documents having as criteria the fact that the document type must have the documenttemplate schema.

A workaround solution is to force the type of documents which will be returned with a query like

 select * from TemplateSource where tmpl:forcedTypes/* <> 'none'

where TemplateSource is the only document type which uses the schema documenttemplate by default