setup an Elastic Cloud cluster for indexing in staging.gitlab.com

as per: gitlab-org&853 (closed) we need to set up an ELK cluster to enable Elasticsearch integration on staging.gitlab.com for gitlab-org group

* [ ] extend our subscription in Elastic Cloud turns out the subscription limit is per deployment so I could create the cluster

• create a cluster for staging
• connect it to staging.gitlab.com (limit to gitlab-org, do not enable searching for now, let indexing run first)
  • check indexing is done, after ~12h:

$ sudo gitlab-rake gitlab:elastic:index_repositories_status
Indexing is 26.30% complete (811186/3084890 projects)

• indexing is done
• if needed, manually trigger indexing: https://docs.gitlab.com/ee/integration/elasticsearch.html#indexing-large-instances
• enable searching with ELK on staging.gitlab.com
• test resizing of the cluster