Fix the save/load cluster configuration on disk and in memory

Tekton Cluster configuration is not persisted on disk (save/load) and is
lost at next reboot. Moreover, configuration update are not properly
sotred in memory, preventing users to access to new configuration added
(other than the default one).

Fixes jenkinsci#106

src/main/java/org/waveywaves/jenkins/plugins/tekton/client/global/TektonGlobalConfiguration.java

@@ -17,7 +17,7 @@
 @Extension
 public class TektonGlobalConfiguration extends GlobalConfiguration {
     private static final Logger logger = Logger.getLogger(TektonGlobalConfiguration.class.getName());
-    private transient List<ClusterConfig> clusterConfigs = new ArrayList<>();
+    private List<ClusterConfig> clusterConfigs = new ArrayList<>();
 
     public TektonGlobalConfiguration(){
         load();
@@ -39,7 +39,8 @@ public static TektonGlobalConfiguration get() {
     @Override
     public boolean configure(final StaplerRequest req, final JSONObject formData) {
         Jenkins.getInstance().checkPermission(Jenkins.ADMINISTER);
-         setClusterConfigs(req.bindJSONToList(ClusterConfig.class, formData.get("clusterConfigs")));
+        setClusterConfigs(req.bindJSONToList(ClusterConfig.class, formData.get("clusterConfigs")));
+        configChange();
         save();
         return true;
     }