In periodic update_available_resource, ResourceTracker.provider_tree
will be updated, and ResourceTracker.assigned_resources will be
populated. When claiming resources, it will get free resources based on
ResourceTracker.provider_tree and ResourceTracker.assigned_resources.
Change-Id: If7f48933db10fcca3b9a05e1e978dfc51f6dabd0
Partially-Implements: blueprint virtual-persistent-memory
Co-Authored-By: He Jie Xu <hejie.xu@intel.com>