Function plan_host 

pub fn plan_host(
    suite: &Suite,
    service: &str,
    my_rank: u8,
    known_roles: &BTreeMap<String, u8>,
    installed_engine: Option<&Path>,
) -> HostPlan

Decide how a process of rank my_rank gets a host for service. A live record whose rank cannot be resolved through known_roles counts as absent. installed_engine is the executable the caller may start when nothing is live; it is used only when it exists.