Parameterize BottleBackend over PlanT (bound to BottlePlan) and
CleanupT (bound to BottleCleanupPlan). DockerBottleBackend declares
itself BottleBackend[DockerBottlePlan, DockerBottleCleanupPlan], which
narrows every method's plan parameter to the concrete type and lets
the six `assert isinstance(plan, DockerBottlePlan)` lines on
launch/cleanup/provision_* go away.
The dict in get_bottle_backend keeps its unparameterized
BottleBackend element type so it can hold heterogeneous backend
specializations.
didericis
cbafbbec5a