Deprecate compute_api_class option in the config

Add a function "get_compute_api_class_name" within nova.compute
that returns nova.compute.cells_api.ComputeCellsAPI if cell is
enabled and this cell is an API cell, and returns compute.api.API
otherwise.
Add an option "cell_type" to the config in group "cells".
Use the default value of "cell_type" (None) for legacy configuration.

Change-Id: I32f5ccf789c657b563c165bfa8244e819b1a79a6
Fixes: bug #1049249
Merge "Deprecate compute_api_class option in the config"