Feature: provide optional threading on ssh actor

This change enables the ssh actor to run commands against multiple hosts in parallel. I took the main idea from here: https://github.com/sprinkle-tool/sprinkle/pull/49 which was never followed up on.

The change is backwards compatible in that one have to explicitly enable it and it only takes effect if there are more than 1 target host. I've tested it and it seems to run fine. Speedups are as expected - there are no noticeable difference between addressing one or many nodes. (To a point, I'm sure)

Sorry about the whitespace changes - I kept them in a separate commit.