Add option to ignore offline node on instance start/stop

In some cases it can be useful to mark as an instance as started
or stopped while its primary node is offline. With this patch,
a new option, “--ignore-offline”, is introduced to “gnt-instance
start” and “… stop”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>