Move calculating new capacity to a function

This is part of the refactoring to fix the
inheritance hierarchy of AutoScaleResourceGroup.
In this case, attempting to move some common
functionality out of the inheritance hierarchy
entirely (perhaps to the heat/scaling module).

Calculating the new capacity while taking
min/max constraints into account, seems like
some common functionality to any type of
scaling.  This way, the extra bounds checking
in handle_update can be eliminated as well.

Change-Id: I0f4c78df15d3d973394ccfa14178941d170c9944
Merge "Move calculating new capacity to a function"