[Dashboards] Switching `pixieCluster` does not update `pixieService` drop-down menu options

[htroisi]

Repro steps:

1. Create a Grafana dashboard by importing the px/service dashboard json.
2. Look at the options in the pixieService drop-down menu.
3. Switch clusters using the pixieCluster drop-down menu.
4. Notice that the options in the pixieService drop-down menu do not update.

This appears to be a problem with multiple dashboards, including px/service and px/pod using Grafana version 7.5.1. However, this does work in the latest version of Grafana.