Do not try to load agentproxies other than netcat over ssh-agent

In JCLOUDS-516, we added support for agentproxies, but only really tested for netcat over ssh. The CLI only supports this, and other agentproxy implementations (e.g. pageant on Windows) fail there since the dependencies aren't even available. See http://markmail.org/thread/oto47qk2kzcdtebb

For now, change this to only try netcat over ssh-agent. If it's possible to include the additional deps in future, we can decide to revert this.

See https://github.com/ymnk/jsch-agent-proxy/pull/23