Make SSH Launch timeout configurable, instead of a hardcoded value of 60...

When the Jenkins master and the docker host are on different networks (continents, actually, in my case :)), and the docker host is slow, the default launch timeout (60s) kills the SSH connection even as the slave is coming up or the job is about to start.

This also triggers a cascade of more failures - the already slow docker host now has to deal with stopping and cleaning up the junked container, even as the plugin tries to spin up a new one.

This pull request adds an UI option to make the SSH Launch Timeout configurable, defaulting to the current 1 minute. Not sure if new options should go to the bottom though.