Get rid of create state in Instance

Get rid of create state in Instance

We don't want to store any state in the Instance object, instead take
advantage of the new mechanism whereby the result of handle_create() is
passed to check_active().

This makes the autoscaling code uglier for now (since it must still store
the instance state in the instance), but this will go away when we move to
using co-routines to create resources in parallel.

Change-Id: Ib11c8da22900453eb9713eba5106859c8a09dbe2