Configurable SSH command for creating tunnel

[tdaff]

Original report by Jiří Kunčar (Bitbucket: jirikuncar, GitHub: jirikuncar).

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Example:

1. research kernel: --tunnel_cmd="ssh -F ~/research/ssh/config"

2. demo kernel: --tunnel_cmd="ssh -F ~/demo/ssh/config"

(no I can't create different rules easily and I don't want to amend my global ssh config)