Make nova-compute import the instance object so that it registers

This makes start/stop work with devstack. Obviously we can't
import all of the things we're going to need in cmd/compute.py,
so we'll have to figure out a better way.

This also makes the failure path for looking up a non-existent
type more helpfully verbose.

Change-Id: I5d25fe27852ed8b7cc6908163a533a49201ea872
Make instance.save() take expected_task_state

This is still plumbed into the old db_api in the same way, but
presented to the rest of nova in a more sane way I think.

Change-Id: Iba7ef4b22bbec1ae55b67c42938dfeab063298c5
Make nova-compute manager use object for start/stop

Change-Id: Idc02e96b0274ffe18cb8cc7cb0923f23b7436eee