Changes in OpenqaJob#find_all_by for more RESTlike caching

 * Goes together with https://github.com/os-autoinst/openQA/pull/90
   (if not, can lead to some unexpected behavior)
 * In addition to the already existing /openqa_jobs/q[iso]=ISO
   Now /openqa_jobs (without filters) also caches (and refreshes if needed)
   the job lists per iso name.
 * It looks more RESTlike since it works coherently with and without filter.
 * It allows to refresh the whole cache in a single call