Currently, cloudkitty only uses Elasticsearch/Opensearch leagacy index, but not datastreams. Datastreams are much nicer, because they handle automation for rotating data to cold storage (if the elastic cluster has multiple types of storage, like hot / warm / cold). This patch adds a configuration option to use datastreams. Change-Id: Iac717c948665effda82a2a87329647c74e3f0667