Add SSH proxy support

[drodil]

Add proxy support for configuration file for global config and server specific config. This means the proxy host, port, username and password. In case proxy is set use it to connect to the target server. This is possible by using the:

proxyClient := ssh.Dial(proxy)
conn := proxyClient.Dial(target) // without config
clientConn, channels, reqs := ssh.NewClientConn(conn, targetConfig)
client := ssh.NewClient(clientConn, channels, reqs)