Refactor to respect env variables in REST API
Reorganize the initialization of the `GraceDB` and `GraceDBBasic` classes so that the command-line tool and the REST API Python module both respect environment variables like `GRACEDB_SERVICE_URL`. As a nice side effect, this change also removes a lot of duplicated code. The main use case is testing GraceDB client code by setting the environment variable `GRACEDB_SERVICE_URL=https://gracedb-test.ligo.org/api/`. This must be addressed before we can make progress on grinch#10.