transport: support LibSSH version 0.6.x instead of 0.4.x

- update rtrlib to require the newest LibSSH versions 0.6.x instead of
  LibSSH 0.4.x
- providing a hostkey for a SSH connection is optional, if no
  hostkey is provided the host authenticity is not verified
- providing a private key is for a SSH connection is optional, if no
  hostkey is provided, rtrlib tries to authenticate with ~/.ssh/id_rsa
rtrclient: replace ssh public key parameter with host key

It's not required anymore to specify a SSH public key for a connection.

Instead of a public key the path to the host key can passed as last
argument (optional).
If it's provided the authenticity of the host is verified.