Puppeth: use ssh-agent instead of reading from file

[lightclient]

Reading ssh keys from file doesn't work for users who rely on ssh-agent to manage their keys (e.g. through gpg). Therefore, it would be great to use ssh-agent to authenticate sessions instead of a key file. It appears the go ssh library has a agent client that would allow for this.

[ligi]

We would like to see a PR with the feature if it is not too complex (max 100 lines diff)

[lightclient]

@ligi ask and ye shall receive.