Support installing the latest version when elasticstack_version: "latest"

[Saeid-Abadi]

The suggestion in PR #368 can be implemented to allow installation of the latest version when elasticstack_version: "latest".

[Saeid-Abadi]

The second way to update the packages to the latest version could be to create installation tasks, as we did for the Beats (e.g. auditbeat) installation.