Enable the use of the ssh-agent if running and available

[Holger Lehmann]

If you have an ssh-agent up and running it is very irritating to add the ssh-key "again" if you have done before during the startup using ssh-askpass.

I would use the same ssh-key to access some remote systems as I have on my system (NFS mounted homedirectory). So the key is already loaded and I guess we could make use of the ssh-agent instead of saving the ssh-key in memory.

I am thinking along the lines of:
​http://twistedmatrix.com/documents/8.2.0/api/twisted.conch.ssh.agent.SSHAgentClient.html

[Antoine Martin]

This is already implemented in r4377

It will be part of the v0.12.6 packages due out later today.