[PSST] Improve "run local" mode

[bbockelm]

Right now, GlideinWMS sets up many environment variables (think PATH, LD_LIBRARY_PATH) before PSST is invoked.

If a site admin wants to invoke PSST themselves, these env vars may be missing.

We should enhance psst.sh to make sure these are given sensible defaults outside the glideinWMS runtime.