Add --name and --region flags to cluster info/down/export commands

[deliahu]

Description

Allow the user to run e.g. cortex cluster down --name=cortex --region=us-west-2 to delete a cluster.

Search for all error messages that mention "cortex cluster down", and add these flags to the error messages if possible.

Motivation

The user won't need access to the cluster configuration file, and can spin down clusters programmatically.