Add a retry path flag to jhack lobotomy

[PietroPasotti]

right now lobotomy has dispatch always exit 0 (on certain events), but maybe we want to tell juju to back off and retry later instead. Todo:

• add a --stop-model-progression, --retry flag or something like that
• implement in lobo-dispatch logic to exit 0 iff that flag is unset (through envvars)