Parameterize a Call with its Context set to a per-host Config.

def parameterize(self, call, host):
        """
        Parameterize a Call with its Context set to a per-host Config.
        """
        debug("Parameterizing {!r} for host {!r}".format(call, host))
        # Generate a custom ConnectionCall that knows how to yield a Connection
        # in its make_context(), specifically one to the host requested here.
        clone = call.clone(into=ConnectionCall)
        # TODO: using bag-of-attrs is mildly gross but whatever, I'll take it.
        clone.host = host
        return clone