Add getConfigGroupSummary API endpoint

Add getConfigGroupSummary API endpoint, which returns a TaskConfig ->
Set<InstanceId> struct for active tasks. This will let us paginate
active tasks, or drop TaskConfig from the getTasksStatus response in the
future.

Testing Done:
./gradlew build
./gradlew FlagSchemaChanges

Bugs closed: AURORA-519

Reviewed at https://reviews.apache.org/r/22429/