[APM UI] Service Maps: No connections callout positioning and display

[formgeist]

Summary

The positioning of the callout is not centered or fullwidth. I prefer to show this fullwidth, but if that's not possible, then at least centered. Another proposal might be to display the callout outside the map frame to make it simpler for us to implement.

Another challenge is what is displayed on the map when we show the callout. Initially it was intended when there was only a single node on the map without any connection information. Now when I see it for a single service instance, I still see the global map underneath.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)