added keepalive function

[mirnujAtom]

Hey Stefan

It turned out that if short timeout frame set on the intermediate host it can close the ssh connection before Terraform sends anything there, so we need to add the keepalive.

• added keepalive
• added keepalive_interval
• corrected error message from my previous pull request

[mirnujAtom]

Hey @stefansundin
Hope you are doing great!

Are there any changes to review/merge the pull request?
The issue I faced with is that Terraform opens the tunnels on when the script started, then, if we've got any long running task in TF (starting an autoscaling group as an example) the intermediate host closes connection before TF reaches the ssh part of the script.

Thanks :)