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.