scylla_node: add datafiles param to ScyllaNode.dump_sstables()

[tchaikov]

it turns out we do have a use case in dtest where the caller of node.run_sstable2json() passes a single sstable to this method. so let's cater its needs.

see compaction_additional_test.py

[tchaikov]

@denesb could you take another look?

[tchaikov]

v2:

• accept a list of sstables.