Move live-migration control flow from scheduler to conductor

This builds on previous patches that moved code to
the conductor and adding the conductor into the control path.

We take care to ensure this works with a local conductor, but
running the code in a greenlet thread, but this will work best
when conductor is run as a standalone service.

The next steps are to move more logic from the compute manager
into the conductor, then to start supporting dealing with
tasks that get stopped half way though execution.

Part of blueprint live-migration-to-conductor
Change-Id: I4120156db1499dfd3ed22095e528787eb73d33a6
Merge "Move live-migration control flow from scheduler to conductor"