Add new compute method for building an instance

The run_instance method on the compute manager handles rescheduling of a
failed build by casting back to the scheduler.  In order to maintain
backwards compatibility while modifying the build process to go through
the conductor a new method is being added to co-ordinate building an
instance.

This is one part of a series of patches to build up the
build_and_run_instance method.

The conductor will be switched over to calling this method once it
reaches a level of consistency with the current run_instance method.

Part of bp query-scheduler

Change-Id: I879805efa4a0a045082c02b10926e1278136b72c
Merge "Add new compute method for building an instance"