Introduce a class inspired by subprocess.Popen that handles
running a command on a remote machine and handling its output.
To separate stdout & stderr streams of a remote command,
they need to be read in parallel, so that one of them doesn't
stall the runner when its buffer fills up. Accomplish this
by using a thread for each stream.
Part of the work for: https://fedorahosted.org/freeipa/ticket/3621
Petr Viktorin
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