Make location of credentials file configurable

[randomir]

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.