Improve Ironic Conductor threading & locks

Ironic Conductor is not consistently using worker threads and
locks as it should.  Instead, it should be using greenthreads
where appropriate to spawn off tasks so that Conductor is freed
up.  This patch corrects do_node_tear_down().

Change-Id: Ia707766855bd7ec47a5af05e119b126504ddc0b1
Partial-Bug: #1285507