BigQuery use environment variable to detect service account json file

[leo-schick]

Currently it is required to set the service_account_json_file_name property for BigQueryDB. Google by default uses the environment variable GOOGLE_APPLICATION_CREDENTIALS to define a path to the service account private key.

I think the service_account_json_file_name property should be made optional and if not set the value should be taken from env. GOOGLE_APPLICATION_CREDENTIALS

See as well: https://cloud.google.com/docs/authentication/getting-started