Buffer ssh output by line to avoid mangling it.

Fixes CHEF-4335 ( http://tickets.opscode.com/browse/CHEF-4335 )

Reads SSH output by slicing up to the first newline and then printing.
Output with no newline is buffered on a per-host basis so that it does
not get interleaved with other hosts' output.
Merge branch 'CHEF-4335'