Support for preemptible tasks via tier configuration

Implementing support for preemptible tasks via tier configuration will involve:

1. Modify scheduler behavior to read "preemptible" value from selected tier and fallback to "production" flag in job configuration.
2. Make the "tier_info" a required command line parameter.
3. Validate tiers.json to ensure at least 1 tier is defined.
4. Update and rename bundled tiers-example.json file to tiers.json.
5. Client change to issue deprecation notice for any continued use of production flag in Job configuration.
6. Add NEWS entry and update docs to announcing+explain the change.