Instead of prompting for login, we should add to our standard NO_AWS_KEYS message https://github.com/mortardata/mortar/blob/master/lib/mortar/local/controller.rb#L47 that you can also login with `mortar login` to automatically sync your AWS keys from Mortar.

Thinking:
```
You have not set AWS access keys, which will often prevent you from accessing input data.  You can either:

- Login to your Mortar account to automatically sync your AWS keys from Mortar ("mortar login")

-  *or*, set your AWS keys via environment variables:

  export AWS_ACCESS_KEY="XXXXXXXXXXXX" 
  export AWS_SECRET_KEY="XXXXXXXXXXXX"

If your script does not need AWS S3 access, you can leave those values as XXXXXXXXXXXX.
```