jenkins_ssh_slave support for launch_timeout

- adds support for the SSH slave advanced setting "launch timeout", 
  which is the timeout that Jenkins will enforce when first trying to 
  connect to a slave; defaults to nil

- backwards compatible support for the ssh-slaves plugin prior to v1.6;
  v1.6 added support for timeouts

- resolves an issue with the jenkins creating a new set of
  ssh credentials with the credentials UUID as the new username
  instead of using the existing credentials id