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.
Yeah, sorry, those should include `ex_` prefix since they are extension methods.