Handle nodepool allocation failure

When a request is either fulfilled or failed, pass it through to
the scheduler which will accept the request (which means deleting
it in the case of a failure) and pass it on to the pipeline manager
which will set the result of the requesting job to NODE_FAILURE
and cause any sub-jobs to be SKIPPED.

Adjust the request algorithm to only request nodes for jobs that
are ready to run.  The current behavior requests all jobs for a
build set asap, but that has two downsides: it may request and
return nodes more aggressively than necessary (if you have chosen
to create a job tree, you *probably* don't want to tie up nodes
until they are actually needed).  However, that's a grey area,
and we may want to adjust or make that behavior configurable later.
More pressing here is that it makes the logic of when to return
nodes *very* complicated (since SKIPPED jobs are represented by
fake builds, there is no good opportunity to return their nodes).

This seems like a good solution for now, and if we want to make
the node request behavior more aggressive in the future, we can
work out a better model for knowing when to return nodes.

Change-Id: Ideab6eb5794a01d5c2b70cb87d02d61bb3d41cce
6 files changed