restapi: backward compatibility for cluster's scheduling policy

The engine implementation for cluster policy has changed, thus
changing the cluster mapper to handle it, without breaking compatibility.

* scheduling_policy_type will be mapped to cluster_policy_name and vice-versa.
* thresholds will be mapped to properties map and vice-versa.

* also, currently in case of 'none' policy, user will see:
<scheduling_policy/>
altered the mapper to show 'none' instead of empty entity.

Change-Id: I4edd7313401670ca225b32d19dd7ad961ccd1b6a
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>