webadmin, core: Prevent arch mismatches in the 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 CPUs compatible with the cluster's current CPU when
  editing clusters;
* Show only clusters with the same architecture as the host when
  editing hosts;

Change-Id: I0033c399061108c4495029ae2bca3f62e75fac68
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>