Fix serial console

Fix serial console

- Binding the console on /dev/console makes it impossible to send
  control characters (eg. ctrl+C). Now that we can, on the ganeti side,
  support a better console mode, with a proper escape character, in
  order to have a working console on the instance side we explicitely
  bind getty to ttyS0, but only when the relevant hypervisor parameter
  is set (of course if that's later changed one has to do the relevant
  change inside the instance, or he'll see init error messages on the
  console)

- Ubuntu has changed to yet another style for init config. Support both.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>