workbench: Improve integration test environment.

Removing too much from the environment causes builds to fail in
Jenkins.  Instead of using a whitelist of environment variables, use a
blacklist of variables that Bundle modifies.
workbench: Improve API server integration launch.

The previous version sometimes tried (and failed) to find the server
pid before rails wrote it.  This provides some limited retry, and
avoids spawning a subshell too.