Make sure that the `keys` file path existence is tested for each server

[bruno-]

For example, if the user specify private key like this:

server "111.112.113.114", user: "deploy", roles: %w{web app db}, keys: "/foo/bar.pem"

Although we have the private_key_exists task, with the key specified like that:

• the key path will not be tested for existence
• we're also not testing if that specific key is loaded to local ssh_agent, and also forwarded to the remote server

This was brought up by the user in #3