Implement SSH passthrough

[kbingham]

Two options (which could both be supported) - a fram command 'ssh' which will connect (only) to the specified target.

ssh target-board@framlab.company.com ssh <ssh options>

And/ or update the ssh authorized_keys to support limiting ssh proxy/jump hosts to the target. That would support both scp and ssh to the target board.
ssh -j target-board@framlab.company.com target-board <ssh options>