[DONOTMERGE] Start ssh tunnel in background

Starting ssh in background mode ensures that `exec_command` will return.

There are some points to discuss:

1. With this change the tunnel will be active until the machine restarts. Do we need something that runs forever or do we need something that runs until the proxy exists?
2. We can use `nohup` [1], but I don't get good results using it. There is also the bash built in `disown`, but I have not tested it because it is not straightforward.
4. Tried using `dtach` and worked better but haven't figured out a way to kill the tunnel later (maybe need to query `ps` to get the pid)
5. A better approach is to create a real daemon so we can start and stop as needed as recommended by the fabric docs [3]

I have submitted this PR so we can discuss and agree upon an approach to use.

[1] http://en.wikipedia.org/wiki/Nohup
[2] http://stackoverflow.com/questions/8775598/start-a-background-process-with-nohup-using-fabric
[3] http://www.fabfile.org/faq.html#why-can-t-i-run-programs-in-the-background-with-it-makes-fabric-hang
As discussed in IRC, it's best to always fail if `status != 0`, regardless of whether an error message is present.