restapi: Cluster parameters override

This feature allows to configure the 'emulated machine' and 'cpu model'
parameters for each VM separately instead of relying on the cluster default.

Change-Id: Ibfe64e713edd43cad33b385699159db534112c02
Bug-Url: https://bugzilla.redhat.com/838487
Feature-page:http://www.ovirt.org/Features/Cluster_parameters_override
Signed-off-by: Eldan Shachar <eshachar@redhat.com>