Add the possibility to add jvmOptions to the slave launch.

Currently the slave.jar is started with just the default java settings. There is no way for the admin to set additional options. I need this to be able to set a git timeout, but it might also be good to have this to set memory limits or other java command line options.

jclouds (cloudstack) slave log from a Jenkins 1.456 instance running with this patch:
Connecting to x.x.x.x on port 22. 
Connected via SSH.
Authenticating as jenkins
Copying slave.jar
Launching slave agent: cd /tmp && java -Dorg.jenkinsci.plugins.gitclient.Git.timeOut=400 -jar slave.jar
<===[JENKINS REMOTING CAPACITY]===>Slave.jar version: 2.33
This is a Unix slave
Evacuated stdout
Slave successfully connected and online