CLOUDSTACK: ex_list_nics, ex_add_nic_to_vm, ex_remove_nic_from_vm + test...

Functionality to list NICS and to add/remove them too.
And some extra attribute maps with respect to nics.
Including unit tests
Some cosmetic (indent) change slipped in this commit as well.
good suggestion, would you prefer to name the method attach_nic_to_node or ex_attach_nic_to_node, since it's a specific extension?