Fixes #5561: Correct SSH key distribution Technique (was non-convergent ...

...when adding keys)

Ticket: http://www.rudder-project.org/redmine/issues/5561

To be merged in 2.11
I think the limits here are a bit low. 10 fields max, maybe, but 4000 bytes is definitely too small. Imagine a 4096-byte key, that's not too far fetched. Can we up this to 10k? "That should be enough for anyone..." as the famous saying goes :D