Fix NPE in DockerSwarmCloud on slave instantiation

INFO [Computer.threadPoolForRemoting [#76691]] hudson.slaves.NodeProvisioner$2.run Image of dockerpriv.nuxeo.com:443/nuxeo/jenkins-slavepriv-jnlp provisioning successfully completed. We have now 31 computer(s)
SEVERE [jenkins.util.Timer [#8]] hudson.triggers.SafeTimerTask.run Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@616a6737 failed
 java.lang.NullPointerException
 at org.jenkinsci.plugins.docker.swarm.DockerSwarmCloud.canProvision(DockerSwarmCloud.java:99)
 at hudson.slaves.NodeProvisioner$StandardStrategyImpl.apply(NodeProvisioner.java:696)
 at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:320)
 at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:62)
 at hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:807)
 at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:72)
 at jenkins.security.ImpersonatingScheduledExecutorService$1.run(ImpersonatingScheduledExecutorService.java:58)
 at java.util.concurrent.Executors$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:1149)
 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
 at java.lang.Thread.run(Thread.java:748)