MANVL-297 [Bugfix] Make sure content repository does not clear complete search index

Add an additional method #clear(Site) to the SearchIndex which allows to clear
only one index. Change the method ContentRepositoryIndex#clear() to use the new
method which ensures that clearing the content repository index does not remove
the indices of all other sites.
Merge pull request #343 from myniva/pr/clean-index-by-site

[Bugfix] Make sure content repository does not clear complete search index