scylla_node: add run_scylla_sstable()

[denesb]

Convenience method allowing the invocation of the scylla-sstable tool, built into the scylla sstable. The method uses the same mechanism for selecting which sstable to work on, as Node.run_sstable2json(). But other than that, there are notable differences, run_scylla_sstable():

• Allows for accessing all scylla-sstable operations, not just dump-data (the scylla-sstable equivalent to sstable2json).
• Allows direct pass-through of command-line arguments to the underlying scylla-sstable invokation. This ensures maximum forward-compatibility, without the need to patch this method each time the underlying code's capabilities change.
• Has a batch mode, which allows for passing all sstables at once to scylla-sstable (which accepts 1 or more sstable args).
• It looses the enumerate_keys parameter.
• Returns the (stdout, stderr) for each underlying invocation, instead of writing the output to a file.

[denesb]

V2:

• Add text=True to subprocess.run() args.
• Use self instead of super().