Acontece que é um bug do plugin jenkins kubernetes. O Kubernetes v1.5.1 tem esse problema ao trabalhar com o plug-in: link .
Picar em toras de jenkins me deu uma dica:
java.lang.NullPointerException
at org.csanchez.jenkins.plugins.kubernetes.KubernetesCloud.addProvisionedSlave(KubernetesCloud.java:592)
at org.csanchez.jenkins.plugins.kubernetes.KubernetesCloud.provision(KubernetesCloud.java:463)
at hudson.slaves.NodeProvisioner$StandardStrategyImpl.apply(NodeProvisioner.java:701)
at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:307)
at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:60)
at hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:798)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:50)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)