vmong5k: remove gateway parameter, use the global g5k_auto_jump setting
The new setting is more flexible, because it allows to force-enable or force-disable the automatic SSH jump host configuration. In contrast, the "gateway" parameter was always enabled when outside G5K.
In addition, it's simpler to have a single setting that applies to both the G5K and VMonG5K providers.
This will break compatibility for experiment code that was using this "gateway" parameter.