webadmin: Adding remaining system tree elements to cache map

Adding remaining system tree elements to the treeItemById map,
this will make all tree elements accessible via
the "SystemTreeItemModel getItemById(Guid id)" method.

Change-Id: I1e92d50ca53891b6c888ce89baf799962d47def3
Signed-off-by: Kobi Ianko <kobi@redhat.com>