Clean up `run_sstable2json` and `run_json2sstable`

There are some improvements to make in `run_sstable2json` and `run_json2sstable` ([found here in `node.py`](https://github.com/pcmanus/ccm/blob/be7d1a9301e25733fd889a5379c7e0c88033cd2b/ccmlib/node.py#L814)):
- provide a `return_output` parameter [like `run_cqlsh`](https://github.com/pcmanus/ccm/blob/be7d1a9301e25733fd889a5379c7e0c88033cd2b/ccmlib/node.py#L710)
- change the lines that print to stdout so they only print as debug lines (this should be done throughout the `run_` methods IMO)
- break out the SSTable printing logic in `run_sstable2json` into its own method
- create ways to operate on strings instead of file handles. This could be implemented as new parameters, but might be better as new methods.