enable file based result caching for all forwarder modules

[simonmeggle]

Refactor gearman forwarder caching: Sakuli should store the result e.g. as JSON in the suite path.
Every forwarder will then use this file in case of retry.
This cache file can be used also for the new "forward-result" mode #141 .

[toschneck]

Also provide a property to set a custom cache folder. This would be needed if the test suite is read-only, like in some docker or openshift mounts