Don't use hardcoded index value in statistics elements

Currently the name of the index is hardcoded but can be overriden:
https://github.com/nuxeo/nuxeo-ui-elements/blob/master/dataviz/nuxeo-document-distribution-chart.html#L174

This means, if a customer changes the index name, he/she will have to override the UI to use the new index.

This is not an ideal scenario.

A more flexible way of doing that could be for instance to detect the index name through the value of elasticsearch.index in the nuxeo.conf through an operation or any other way.