[WIP] feature: support argo retry

[dhpikolo]

This PR, modifies the step cli command to infer retry_count from flow_datastore class. Looks like this class, holds all the information about underlying datastore and run artifacts.

• Adds a flow_datastore class method that infers latest done attempt of a task.
• Remove MF_ATTEMPT from mflog.save_logs python script and infer attempt from the datastore.

Resolves: https://github.com/Netflix/metaflow/issues/2278

[dhpikolo]

A known limitation for this solution would be:

• Retrying more than 6 times (or in current case, hitting argo retry button twice) times results in metaflow exception

We are planning to address this limitation by making MAX_ATTEMPTS configurable via envvar.

• https://github.com/Netflix/metaflow/pull/2279