Pass assigned nodes to the launcher

This fleshes out the nodepool stub a little more.  It passes some
node information to the launcher after node requests have been
fulfilled.  It also corrects some logic errors in the node request
framework.  It moves data structures related to node requests into
the model.  Finally, it adds nodes to the configuration of some
tests to exercise the system, and adds a test to verify the correct
node is supplied on a job that has a branch variant.

Change-Id: I395ce23ae865df3a55436ee92d04e0eae07c963a
9 files changed