Resolve naming conflicts

[j3t]

The jobcacher-plugin provides a pipeline step named cache already. Since this plugin provides also a pipeline step named cache, this leads to a conflict if you want to use both plugins at the same time.

To solve this issue, the cache step is renamed to fileCache. According to the pipeline-step-reference, the fileCache step not exists yet. To avoid breaking changes, the old cache step function will still be available, at least for a certain period of time, but it prints out a warning message if it is used.