Remove 'get_host_stats' virt driver API method

In Havana the following change was merged:

  commit 680cc7becc38178f47e13b3aa11c8544b1949de7
  Author: Jake Liu <jake@unitedstack.com>
  Date:   Mon Sep 2 08:56:51 2013 -0400

    Remove _report_driver_status from compute/manager.py

    After the blueprint no-compute-fanout-to-scheduler, there is no
    need for nova compute to fanout its capabilities to nova scheduler,
    so we can remove _report_driver_status as it is no use now.

This removed the last use of the 'get_host_stats' method outside
of the virt drivers themselves. There was a default implementation
of the 'get_available_nodes' method which relied on 'get_host_stats'
but that is trivially implementable directly in each driver.

There is thus no compelling reason to continue to have a public
'get_host_stats' method with such ill-defined (read: totally
undocumented) return dict.

Change-Id: I7d18fa1ab3a2f04c8b514aa4523a37a5dfb5f33c
Merge "Remove 'get_host_stats' virt driver API method"