[Maps] Document source top hits - can not split entity on scripted field

[nreese]

Follow these steps to view the problem

1. Create Document source from web logs sample data set
2. Set time picker to last 24 hours
3. configure top hits to split entity on hour_of_day
4. Notice how there are now no results

This is because the aggregation is not configured correctly. The aggregation sets the field parameter when it should set the script parameter here.

This is not a regression, it's a bug that has existed since top hits was implemented.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)