Start rework of attribs to let them be role driven.

Among the changes:

* Declare default attribs in crowbar.yml, either in a top-level attribs
  section or as part of a role.
* Enhance the attrib type finding model to allow for jig-specific
  customizations.
* Allow roles to declare that they want specific attributes.  Nothing
  uses this now, but it will wind up replacing the current attribute
  gathering logic that the jigs use.
Merge pull request #791 from VictorLowther/pull-req-master-a5df278b8c055255c255e395c0bbfbf2cf28a449

Start rework of attribs to improve data isolation during annealing [1/3]