add launch_timeout as an optional attribute to the jenkins_ssh_slave LWRP

- 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

- add debug trace statements to the executor; useful for debugging a
  converge where the groovy CLI calls are not behaving
  as expected

- backwards compatible support for the ssh-slaves plugin prior to v1.6;
  v1.6 added support for timeouts and also introduced the bug with
  loading credentials by UUID