Consolidate input environment variables used to collect required and optional metadata for the agent

[portante]

Wow; lots of these, with namespace all over the place. This is definitely something we should aspire to improve in the the future!

Originally posted by @dbutenhof in #2181 (comment)

[webbnh]

Yes, but...what kind of latitude do we have for making changes to them? Aren't we almost completely restricted by binary and user compatibility? (I.e., aren't nearly all of these documented, despite their variety of styles?)

[portante]

As we productize, we can declare a cut-over date so that we use all "pbench" or "PBENCH_*" environment variables, adding a shim later which would issue a warning, about being dropped in the 1.0 version.