webadmin: fix cluster creating in guide

When creating a cluster in the datacenter guide, policy units
configuretion was not saved.

added missing policy unit mapping from ClusterModel to VDSGroup

Change-Id: I7cc47558a06e4e0c000219ab2b8b34d9fac9f5a8
Bug-Url: https://bugzilla.redhat.com/1001182
Signed-off-by: Noam Slomianko <nslomian@redhat.com>