Allow randomizing cloud selection

When multiple Docker clouds are defined, the plugin will always create
agents from the first available cloud, not allowing to distribute the
load on all the clouds that are defined.

Possible solutions to this issue are:
- Docker Swarm Standalone, which is deprecated
- Docker Engine Swarm API, which is not supported by docker-plugin.

This changes adds a setting that enables randomizing the order in which
Docker clouds are tried when creating a new agent.

src/main/java/io/jenkins/docker/DockerGlobalConfiguration.java

@@ -0,0 +1,29 @@
+package io.jenkins.docker;
+
+import jenkins.model.GlobalConfiguration;
+import org.kohsuke.stapler.DataBoundSetter;
+import hudson.Extension;
+import hudson.ExtensionList;
+
+@Extension
+public class DockerGlobalConfiguration extends GlobalConfiguration {
+    public static DockerGlobalConfiguration get() {
+        return ExtensionList.lookupSingleton(DockerGlobalConfiguration.class);
+    }
+
+    private boolean randomizeCloudsOrder = false;
+
+    public DockerGlobalConfiguration() {
+        load();
+    }
+
+    public boolean getRandomizeCloudsOrder() {
+        return randomizeCloudsOrder;
+    }
+
+    @DataBoundSetter
+    public void setRandomizeCloudsOrder(boolean value) {
+        randomizeCloudsOrder = value;
+        save();
+    }
+}

src/main/java/io/jenkins/docker/FastNodeProvisionerStrategy.java

@@ -6,6 +6,7 @@
 import static hudson.slaves.NodeProvisioner.StrategyDecision.PROVISIONING_COMPLETED;
 import static java.util.logging.Level.FINE;
 import static java.util.logging.Level.FINEST;
+import static java.util.logging.Level.WARNING;
 
 import com.nirima.jenkins.plugins.docker.DockerCloud;
 import edu.umd.cs.findbugs.annotations.NonNull;
@@ -16,7 +17,9 @@
 import hudson.model.queue.QueueListener;
 import hudson.slaves.Cloud;
 import hudson.slaves.NodeProvisioner;
+import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.logging.Logger;
 import jenkins.model.Jenkins;
 
@@ -36,12 +39,22 @@ public StrategyDecision apply(@NonNull NodeProvisioner.StrategyState state) {
         if (Jenkins.get().isQuietingDown()) {
             return CONSULT_REMAINING_STRATEGIES;
         }
+        ArrayList<DockerCloud> dockerClouds = new ArrayList<DockerCloud>();
         for (Cloud cloud : Jenkins.get().clouds) {
             if (cloud instanceof DockerCloud) {
-                final StrategyDecision decision = applyToCloud(state, (DockerCloud) cloud);
-                if (decision == PROVISIONING_COMPLETED) {
-                    return decision;
-                }
+                dockerClouds.add((DockerCloud) cloud);
+            }
+        }
+        if (DockerGlobalConfiguration.get().getRandomizeCloudsOrder()) {
+            LOGGER.log(WARNING, "Randomizing cloud order");
+            Collections.shuffle(dockerClouds);
+        } else {
+            LOGGER.log(WARNING, "Not randomizing cloud order");
+        }
+        for (DockerCloud cloud : dockerClouds) {
+            final StrategyDecision decision = applyToCloud(state, cloud);
+            if (decision == PROVISIONING_COMPLETED) {
+                return decision;
             }
         }
         return CONSULT_REMAINING_STRATEGIES;

src/main/resources/io/jenkins/docker/DockerGlobalConfiguration/config.jelly

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form">
+    <f:section title="Docker Plugin Configuration">
+        <f:entry title="Randomize clouds order" field="randomizeCloudsOrder"
+            description="During node provisioning, randomize the order clouds are tried.">
+            <f:checkbox/>
+        </f:entry>
+    </f:section>
+</j:jelly>