LUSetInstanceParams: Fix nic handling

CheckArguments:
  Use constants.VALUE_NONE rather than hardcoding the string "none"
  If we're adding a nic fill the nic_dict with default values
  Check if the mac is syntactically valid, if we have one
  Don't allow the mac to be 'auto' when modifying a nic

CheckPrereq:
  Check that bridge and mac if present in the dict are not None
    (before this wasn't handled at all)
  Generate the nic mac address here if demanded

Exec:
  Do not generate nics and macs

Reviewed-by: iustin