Relateddocs plugin only searches documents having the 'preview' value in 'hippo:availability' property

The relateddocs plugin query documents with related content based on the 'rep:similar' predicate. It only searches for document having hippo:availability='preview'.

Reproduction path:

1. Generate an archetype project.
2. Add blog document type, add relateddocs plugin. Rebuild the project.
3. Add a relateddoc field to the blog document type.
4. Configure blog RSS field to import blogs from an RSS, e.g., http://www.onehippo.com/en/rss/
5. In CMS, open an imported blog:
   • Expectation: The relateddoc field display all related docs
   • Actual: it does not display related blogs because imported documents do not contain 'preview' in 'hippo:availability'
6. Edit some imported blogs: close & save. The relateddocs field contains these documents.