Setup snapshot for s3 plugin

[sebgl (Sebastien Guilloux)]

I'm not very familiar with the s3 repository, but by looking around I guess:

• either there is a problem with secure settings themselves (wrong value or not set correctly), see Error while creating snapshot to s3 repository
  Can you double-check your s3.client.default.access_key and s3.client.default.secret_key are correct? Note their value should be base64-encoded in the Kubernetes secret.

• either the host on which Elasticsearch is running cannot reach aws metadata server, see https://stackoverflow.com/questions/58378329/aws-instance-metadata-for-iam-is-not-found