Metadata api doesn't support ES6.X

[adrienyhuel]

Elastalert create multiple writeback indexes for ES > 6 : https://github.com/Yelp/elastalert/blob/master/elastalert/create_index.py -> for ES < 6, documents are in a single index, seprated by document type -> for ES >= 6, document are in multiple indexes

But in the metadata api, the handler search only in the writeback index, filtering by document type. Then it is only compatible with ES < 6.