core: Fix problem with changing host cluster

The VDS entity vds_group_id property was changed to vdsGroupId, in the
canDo action of ChangeVDSCluster the field change validation was hard coded
to the old property name thus failing on host cluster change even if the
host is in maintenance state, changed the check to use the new field name

Change-Id: Ia5789c931e69bc2ed5f7840d8503dcf843a102ed
Signed-off-by: Tal Nisan <tnisan@redhat.com>