webadmin: Filter clusters by arch in FE

This patch introduces some changes in the frontend in order to prevent
the user from making mistakes and receive errors from the backend
commands. These changes include:

* Show only clusters with the same architecture as the VM, Pool or
  Template when editing them;
* Show only clusters with the same architecture as the VM used as a
  model when creating a template;
* Hide clusters with undefined architecture in the 'New VM' and
'New Pool' dialogs.

Change-Id: I89df0de70a26379642a09e765631d3abf3200339
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>