open-tunnel leaves juju unable to connect to an environment if local IP changes

[Clint Byrum]

Test case:

1. Establish a connection to network A
2. juju open-tunnel to a bootstrapped ec2 environment
3. Change network connection to network B, with a different IP
4. ctrl-C or kill to end the juju open-tunnel command.
5. try to issue other juju commands, they will hang connecting through the tunnel

I think because of the way open-tunnel runs ssh, the ssh stays open indefinitely trying to send packets and failing. Even after open-tunnel is killed, the underlying ssh will remain:

clint 3756 0.0 0.1 48296 6020 pts/0 S 15:52 0:00 ssh -T -o ControlPath /home/clint/.juju/ssh/master-%r@%h:%p -o ControlMaster auto -o PasswordAuthentication no -Llocalhost:44888:localhost:2181 <email address hidden>

This connection should probably use some kind of keepalive and die on its own if the local IP is changed. If nothing else, the open-tunnel command should be killing the ssh process when it is killed.