webadmin: CPU Type and CPU Architecture fields in Cluster dialog fix

* CPU Architecture listbox is placed right above the CPU Type because
  they are related. (ClusterPopupView.ui.xml)

* CPU Architecture field doesn't disappear when CPU Type is selected.
  (Bug fix). (ClusterPopupView.java)

* CPU Architecture filters content of CPU type listbox.
  (ClusterModel.java, FilteredListModel.java)

Bug-Url: https://bugzilla.redhat.com/1159777
Change-Id: I4c0452426f58f09703382495a9527fdbe859d80e
Signed-off-by: Jakub Niedermertl <jniederm@redhat.com>