Arvados

Following on the api_for_instance logic in arv-copy, it would be nice to generalize this and use it in our other tools.

We want behavior like:

• If the target cluster can be inferred (e.g. there's an Arvados uuid in the input somewhere) then the tool can choose the right credentials by default (this is how arv-copy works currently).
• Instead of ARVADOS_API_HOST and ARVADOS_API_TOKEN we could have single environment variable like ARVADOS_CLUSTER (or ARVADOS_INSTANCE) that is just the cluster id
• Getting credentials from workbench and saving them locally should be streamlined (e.g. #21847)
• It would convenient to have the default settings.conf symlink to the config file named by cluster id.

Also, it could be helpful to check token validity using api.api_client_authorizations().current() and providing a standardized error message instead of failing on first use.

We might want to put this logic into the default constructor (e.g. arvados.api()) with a new optional parameter like cluster_id: str as well as looking for ARVADOS_INSTANCE in the environment. This would make the new functionality available for everyone, while the behavior of inferring the right cluster can be phased in to individual tools.