.alerts-* auto-expand replicas 0-1

[kobelb]

Currently, the .alerts-* are being created with with number_of_replicas: 1. This will cause clusters that only have a single Elasticsearch node to always appear unhealthy because they can't create a replica. Instead, we should change the .alerts-* indices to have auto_expand_replicas: 0-1 to make clusters with a single Elasticsearch node healthy, but expand to a single replica as soon as there is a second node. This is similar to how we handle the .kibana_* indices.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[nerophon]

@doakalexi thanks for working on the PR to fix this. Is there any possibility this could please also be back-ported into the next 7.17 release? Thank you. cc @kobelb .