Align report metrics field names

[elkargig]

It seems that rally uses slightly different variations of field names for report metrics and this can cause some confusion.

Describe the feature:
For example metrics documentation reports indexing_total_time but this is sent as total_time in Elasticsearch reporting data store.

The field names are also referenced in summary report documentation

It would be nice if the field names appearing in the report documentation are also present in Elasticsearch reporting data store.