core, webadmin: Cluster and architecture related validations

This patch introduces the following validations:
* Reports an error if the cluster does not support the CPU from a
  host;
* Prevents a host from changing into a cluster with a different
  architecture;
* Does not allow the architecture to be changed if there are hosts or
  VMs inside the cluster;

Change-Id: If19c3ee99f5ef17721bb4111ddfb48977d1b578b
Signed-off-by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>