Bootstrapping juju hangs in ssh to machine-0

[Peter Sabaini]

Bootstrapping juju hangs on a machine here:

```
sudo snap install openstack
sunbeam prepare-node-script | bash -x && newgrp snap_daemon
sunbeam cluster bootstrap --accept-defaults
#... hangs in deploying juju
```

I can see an ssh process trying to connect to the local machine during the hang.

I believe this is because the prepare-node-script configures a passwordless ssh with a key in ~/.ssh/id_rsa but only if that key doesn't already exist. This machine already has a non-passwordless key configured which is supposed to be driven via ssh-agent, however the ssh-agent conn seems to fail for sunbeam, so the authentication never completes