separate CORE control IP and data tunneling IP

```
It is possible to connect to CORE port 4038 from another machine via SSH port
forwarding.

The GUI may be configured to connect to e.g. localhost:54038 which forwards to
myserver:4038. When GRE tunnels are built, then localhost is used as the tunnel
endpoint (instead of myserver). This should be a separate session option (?) or
server config item to specify mappings between servers and IP addresses that
will be used for tunnel endpoints.

This would enable a remote GUI to connect via SSH over port forwarding while
the data connection used for GRE tunnels is on a local network.

Similarly, double-clicking on a node for shell access should be configurable,
e.g. ssh to a specific host:port combination.
```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 16 Apr 2013 at 2:49