Allocation routines: return list of resource stats

Currently, the allocation routines (iterateAlloc and tieredAlloc)
return only the final state of the cluster and the list of allocated
instances. For better visibility in how the cluster resources change,
we compute and return a list of cluster resources at each step, which
should be a good indicator of the cluster state. The cost of the
computation for the new resource when not used is undetectable (< 1%).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>