Get rid of the old config registration mechanism.

It was hardly used, and is now superceded by the new options system.

Moved a bunch of accesses of pants_workdir from config to new options.
There are still several such accesses that will take more work, as
they are in code locations with no reference to an Options instance.

To do so, made bootstrap option values available during option registration.
This uses function properties, which actually seemed like a reasonable
solution here.  Yay python for making everything an object.

Testing Done:
CI passes.

Reviewed at https://rbcommons.com/s/twitter/r/1385/