System Overview rotation notification should label which index set (logging enhancement)

Alone, this log event is ambiguous:
`Number of indices (169) higher than limit (168). Running retention for 1 indices.
`
## Expected Behavior
Adding impacted index set would provide context:
`index Number of indices (169) higher than limit (168). Running retention for 1 indices.`
or
`[palo-traffic] Number of indices (169) higher than limit (168). Running retention for 1 indices.`

## Current Behavior
Individual log event does not have any pointers to the actual impacted index set. Nearby log events have this information.
```
2019-05-03T01:52:41-05:00 SystemJob [org.graylog2.indexer.indices.jobs.OptimizeIndexJob] finished in 3079797ms.
2019-05-03T01:35:49-05:00 SystemJob <0fefacb0-6d69-11e9-be00-24b6fdf4c80c> [org.graylog2.indexer.indices.jobs.OptimizeIndexJob] finished in 1988056ms.
2019-05-03T01:02:42-05:00 SystemJob [org.graylog2.indexer.indices.jobs.SetIndexReadOnlyAndCalculateRangeJob] finished in 2215ms.
2019-05-03T01:02:41-05:00 Optimizing index .
2019-05-03T01:02:41-05:00 Flushed and set to read-only.
2019-05-03T01:02:38-05:00 Running retention strategy [org.graylog2.indexer.retention.strategies.DeletionRetentionStrategy] for index
2019-05-03T01:02:36-05:00 Number of indices (169) higher than limit (168). Running retention for 1 indices.
```

## Possible Solution
It might make more sense to standardize the system logs to prepend the impacted index for all actions.

```
2019-05-03T01:52:41-05:00 [palo-traffic] SystemJob [org.graylog2.indexer.indices.jobs.OptimizeIndexJob] finished in 3079797ms.
2019-05-03T01:35:49-05:00 [palo-traffic] SystemJob <0fefacb0-6d69-11e9-be00-24b6fdf4c80c> [org.graylog2.indexer.indices.jobs.OptimizeIndexJob] finished in 1988056ms.
2019-05-03T01:02:42-05:00 [palo-traffic] SystemJob [org.graylog2.indexer.indices.jobs.SetIndexReadOnlyAndCalculateRangeJob] finished in 2215ms.
2019-05-03T01:02:41-05:00 [palo-traffic] Optimizing index .
2019-05-03T01:02:41-05:00 [palo-traffic] Flushed and set to read-only.
2019-05-03T01:02:38-05:00 [palo-traffic] Running retention strategy [org.graylog2.indexer.retention.strategies.DeletionRetentionStrategy] for index
2019-05-03T01:02:36-05:00 [palo-traffic] Number of indices (169) higher than limit (168). Running retention for 1 indices.
```

## Context
Visibility improvement in system overview

* Graylog Version: 3.0.1