enable withCredentials in elasticsearch basic auth

Currently cookies are passed in requests to graphite and not passed to elasticsearch. The reason is that `withCredentials` is set to `true` in graphite datasource (see [here](https://github.com/grafana/grafana/blob/master/src/app/services/graphite/graphiteDatasource.js#L202)), but not set in ES datasource.

This pull request fixes the behavior.