Refactor to respect env variables in REST API
requested to merge leo-singer/gracedb-client:refactor-to-respect-environment-variables-in-rest-interface into master
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.