`$._config.ingester.statefulset_replicas` not honoured for blocks storage ingesters

[eamonryan]

In https://github.com/grafana/cortex-jsonnet/blob/master/cortex/ingester.libsonnet#L92-L94 we can see:

statefulSet.new('ingester', $._config.ingester.statefulset_replicas

So, the number of replicas comes from the config file value.

However, in https://github.com/grafana/cortex-jsonnet/blob/master/cortex/tsdb.libsonnet#L87-L88 we can see

newIngesterStatefulSet(name, container)::
    statefulSet.new(name, 3, [

So if using tsdb.libsonnet you get ingesters hardcoded to 3, which seems odd.

cc @pracucci as filing this per his request.