Call scheduler for run_instance from conductor

This prepares for having the conductor query the scheduler for a list of
hosts to build an instance on.  In order to accomplish this the API
sends a build_instances message to conductor, or spawns a greenthread in
local mode, rather than sending a message to the scheduler.  This is
being done so that conductor can handle the orchestration of long
running tasks like spawning instances.  By making this move, the API is
free to return to the caller while conductor queries the scheduler for a
host to provision to.

In the case of cells the build_instances message first goes to the
cells scheduler in order to pick a cell to send it to, and then it is
sent to the conductor in that cell.

Part of bp query-scheduler
Change-Id: I4539888e78ebdbb8cef6647273b959a012280110
Merge "Call scheduler for run_instance from conductor"