Prevent over-provisioning

Jenkins can keep on asking for more executors even though it has enough
slaves promised to it that will satisfy demand once they come online.
We already keep track of the number of slaves that we are in the process
of creating but have yet to give to Jenkins, so if we include that count
when determining demand, we can ensure we do not provision more
containers than permitted.

src/main/java/com/nirima/jenkins/plugins/docker/DockerCloud.java

@@ -288,6 +288,22 @@ public synchronized Collection<NodeProvisioner.PlannedNode> provision(final Labe
             final List<DockerTemplate> templates = getTemplates(label);
             int remainingWorkload = numberOfExecutorsRequired;
 
+            // Take account of the executors that will result from the containers which we
+            // are already committed to starting but which have yet to be given to Jenkins
+            for ( final DockerTemplate t : templates ) {
+                final int numberOfContainersInProgress = countContainersInProgress(t);
+                final int numberOfExecutorsInProgress = t.getNumExecutors() * numberOfContainersInProgress;
+                remainingWorkload -= numberOfExecutorsInProgress;
+            }
+            if ( remainingWorkload != numberOfExecutorsRequired ) {
+                final int numberOfExecutorsInProgress = numberOfExecutorsRequired - remainingWorkload;
+                if( remainingWorkload<=0 ) {
+                    LOGGER.info("Not provisioning additional slaves for {}; we have {} executors being started already", label, numberOfExecutorsInProgress);
+                } else {
+                    LOGGER.info("Only provisioning {} slaves for {}; we have {} executors being started already", remainingWorkload, label, numberOfExecutorsInProgress);
+                }
+            }
+
             while (remainingWorkload > 0 && !templates.isEmpty()) {
                 final DockerTemplate t = templates.get(0); // get first