Get rid of the old config registration mechanism.

Review Request #1385 - Created Nov. 21, 2014 and submitted

Benjy Weinberger
jsirois, patricklaw, travis, zundel

It was hardly used, and is now superceded by the new options system.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

CI passes.

Eric Ayers
Benjy Weinberger
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted as 82b8aff167e3528fc2a559e81aeb8863328162fb.