public List<Server> getEligibleServers(List<Server> servers) {
        return getEligibleServers(servers, null);
    }
Get servers filtered by this predicate from list of servers. Load balancer key
is presumed to be null. 

@see #getEligibleServers(List, Object)
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/AbstractServerPredicate.java