webadmin: OVF import for multiple architectures

* This patch prevents the user from import VMs and templates with
  different architectures into the same cluster. This is done by
  showing a warning when there are selected VMs with different
  architectures in the Import VM and Import Template dialog.
* Also, when the Import VM dialog is shown the only clusters displayed
  in the dialog are the ones compatible with the architecture of the
  selected VMs and templates.

Change-Id: I77c5bee8d71dd394fd7d4b51645bb0b296788780
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>