Add @SingleValue on hosts config

Author: adrienlauer

src/it/resources/application.yaml

@@ -8,5 +8,4 @@
 
 elasticSearch:
   clients:
-    client1:
-      hosts: localhost
+    client1: localhost

src/main/java/org/seedstack/elasticsearch/ElasticSearchConfig.java

@@ -10,6 +10,7 @@
 import org.elasticsearch.plugins.Plugin;
 import org.hibernate.validator.constraints.NotEmpty;
 import org.seedstack.coffig.Config;
+import org.seedstack.coffig.SingleValue;
 
 import java.util.ArrayList;
 import java.util.Collections;
@@ -35,6 +36,7 @@ public ElasticSearchConfig addClient(String name, ClientConfig config) {
 
     public static class ClientConfig {
         @NotEmpty
+        @SingleValue
         private List<String> hosts = new ArrayList<>();
         private Properties properties = new Properties();
         private Set<Class<? extends Plugin>> plugins = new HashSet<>();