Add check for option :bus_type in Storage#connect

This pull request adds a check in `Storage#connect` which only allows valid values for the `:bus_type` option (according [ProfitBricks API 1.2 documentation](https://www.profitbricks.com/de/en/iaas-developer-center/profitbricks-api/apidoc/APIDocumentation.html?ConnectStorage.html)) and corrects the documentation for this method.