vmalert only supports a single datasource for querying metrics and
managing alerts. Because of that, we need two instances to manage alerts
for both VictoriaLogs and VictoriaMetrics.
This is strongly inspired by the change made to Redis, i.e. a new
`instances` option was introduced with each option inside it.
With `mkRenamedOptionModule` it's ensured that existing configurations
still evaluate to the same result.
