feat(ssh): Use ssh-agent instead of private_key

[gnuletik]

This fixes the following issue when using a password protected private key :

password protected keys are not supported. Please decrypt the key prior to use.

[stephenmoloney]

I also use ssh-agent but I didn't make a merge request as it forces others to do the same.
If this could be made using a conditional and a terraform variable I think that would be a bit better so users could opt for ssh-agent according to the user's terraform vars.

[gnuletik]

Hi ! @stephenmoloney ! Thanks for your fast answer.

Yeah, that's what I thought.
I'll add a variable to configure it if that's possible (I'm new to terraform).
I'll update the PR in the coming days.