Reduce DB load caused by heal instance info cache

Load system metadata with instance fetches in the
_heal_instance_info_cache periodic task.  This will eliminate
DB queries to lazy load the system metadata in _get_instance_nw_info.

(lazy loading happens in the Instance object's obj_load_attr method)

bug 1223969

Change-Id: I835fc73ef3a92885cec174e1533db10e28752fe1