Don't ignore environment variables pointing to valid configuration files

Fixes saltstack/salt-cloud#661

`salt.config.load_config()` cannot be hard-coded to use only the master configuration file as the default configuration file path to know if the user has passed any configuration file in the CLI tool or not.
To fix this, `default_path` was added. This way, the minion config or client config or even the salt cloud config can say which is the expected default path of the configuration file and properly load config from environment variables.