Fix for JENKINS-51286: container cap limit

Issue : Setting container cap limit doesn't work correctly without Jenkins restart. Once user set container cap limit configuration for given namespace, kubernetes plugin consider all pods/containers from the namespace which are not started by the plugin (ideally it should query slave pods/container which are started by the plugin).
This patch fixes this issue by setting default slave labels to query pods/containers while checking the container cap limit.

src/main/java/org/csanchez/jenkins/plugins/kubernetes/KubernetesCloud.java

@@ -78,8 +78,8 @@ public class KubernetesCloud extends Cloud {
     private static final Logger LOGGER = Logger.getLogger(KubernetesCloud.class.getName());
 
     public static final String JNLP_NAME = "jnlp";
+
     /** label for all pods started by the plugin */
-    @Deprecated
     public static final Map<String, String> DEFAULT_POD_LABELS = ImmutableMap.of("jenkins", "slave");
 
     /** Default timeout for idle workers that don't correctly indicate exit. */
@@ -325,7 +325,7 @@ public int getConnectTimeout() {
      * Labels for all pods started by the plugin
      */
     public Map<String, String> getLabels() {
-        return labels == null ? Collections.emptyMap() : labels;
+        return (labels == null || labels.isEmpty()) ? DEFAULT_POD_LABELS : labels;
     }
 
     public void setLabels(Map<String, String> labels) {

src/main/java/org/csanchez/jenkins/plugins/kubernetes/PodTemplate.java

@@ -351,6 +351,7 @@ public Map<String, String> getLabelsMap() {
                 builder.put(label == null ? DEFAULT_ID : "jenkins/" + label.getName(), "true");
             }
         }
+        builder.putAll(KubernetesCloud.DEFAULT_POD_LABELS);
         return builder.build();
     }
 

src/test/java/org/csanchez/jenkins/plugins/kubernetes/KubernetesCloudTest.java

@@ -2,6 +2,7 @@
 
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.Map;
 
 import jenkins.model.JenkinsLocationConfiguration;
 import org.csanchez.jenkins.plugins.kubernetes.volumes.EmptyDirVolume;
@@ -12,6 +13,7 @@
 import org.jvnet.hudson.test.JenkinsRule;
 
 import static junit.framework.TestCase.assertEquals;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.fail;
 
 public class KubernetesCloudTest {
@@ -76,5 +78,17 @@ public void getJenkinsUrlOrDie_UrlInLocation() {
         assertEquals("http://mylocation/", cloud.getJenkinsUrlOrDie());
     }
 
+    @Test
+    public void has_default_labels_for_slave_pods() {
+        // GIVEN
+        KubernetesCloud cloud = new KubernetesCloud("name");
+
+        // WHEN
+        Map<String, String> defaultLabels = cloud.getLabels();
+
+        // THEN
+        assertNotNull(defaultLabels);
+        assertNotNull(defaultLabels.get("jenkins"));
+    }
 
 }