Wait indefinitely when launching slave agent with a timeout of zero

Summary:
The docs claim that a timeout of zero means forever, but sometimes it
turns out to really mean zero. We correct that here.

Test Plan:
Ran the new code on our Jenkins installation, saw that we didn't get the
timeout message where Jenkins waits for only about 5 seconds before
claiming that it failed to launch the slave agent.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D10554

src/main/java/hudson/plugins/ec2/ssh/EC2UnixLauncher.java

@@ -211,7 +211,7 @@ private Connection connectToSsh(EC2Computer computer, PrintStream logger) throws
         while(true) {
             try {
                 long waitTime = System.currentTimeMillis() - startTime;
-                if ( waitTime > timeout )
+                if ( timeout > 0 && waitTime > timeout )
                 {
                     throw new AmazonClientException("Timed out after "+ (waitTime / 1000) + " seconds of waiting for ssh to become available. (maximum timeout configured is "+ (timeout / 1000) + ")" );
                 }