Make location of credentials file configurable

Currently, `dwave.cloud.auth.flow.AuthFlow` will use `Credentials` backed by a file (created) next to the most-specific dwave config file. Typically, config file will be in `~/.config/dwave/dwave.conf`, and credentials will be stored right next to it in `~/.config/dwave/credentials.db`.

If no config file is present on disk, credentials are stored in the default user config file location (see `dwave.cloud.config.get_default_configfile_path`).

We need to either generalize `DWAVE_CONFIG_FILE` semantics to include credentials, or introduce a new variable.