Known hosts authorization

[danfruehauf]

If the destination machine is not in your known_hosts file, things would fail. Suggesting the following:

• When saving the dialog file, ask the user if he wants to authorized that host in known_hosts
• Have $HOME/.ssh/known_hosts passed to nm-ssh-service, rather than using the one at /root/.ssh/known_hosts

[danfruehauf]

Some of the pain was taken away after: https://github.com/danfruehauf/NetworkManager-ssh/commit/9375210f26d6601217e219e69b197605cdc51448

Now using user's known_hosts file instead of root's.