After upgrading to 1.37, none of my spot instances would register with jenkins as a node.

1.37 introduced the ability to add a prefix command before the launchstring for EC2UnixLaunchers (https://github.com/jenkinsci/ec2-plugin/pull/224), however if one is not using a prefix command, it breaks the launch string somehow.  The logs were showing two spaces in front of the command. 

INFO: Launching slave agent (via Trilead SSH2 Connection):  java  -jar /tmp/slave.jar

And when I would ssh into the slaves, there was no slave.jar running.

So I patched out the prefix string here: 

https://github.com/jenkinsci/ec2-plugin/blob/d5fbd012bf0c18e85ee85653694cf6c51ad4cd3b/src/main/java/hudson/plugins/ec2/ssh/EC2UnixLauncher.java#L218

To remove the prefix part:

String launchString = "java " + (jvmopts != null ? jvmopts : "") + " -jar " + tmpDir + "/slave.jar";

then I repackaged the plugin, upgraded to my bespoke version, and everything was working again.

Im not sure if this is a weird bug with the trilead ssh client that doesnt like commands to be prefixed with spaces or what, but at least that fixed the issue allowing me to use 1.37