Arduino yun as ssh client
You need to install package openssh-client: it's available in the package list. See this tutorial about how to install optional packages.

You can call ssh on the Linux side using Process from the Bridge Arduino library.
Best is when you execute single or grouped commands using a syntax as:

ssh user@your_other_host 'echo 1; echo 2'


Managing an open ssh session is possible but I think may be troublesome, so avoid it if you can.