[Sky Launch] SSH to Machine during first run of sky launch

[michaelzhiluo]

It looks like the steps that allow the user to authenticate and access the machine (modifications to ~/.ssh/config) are done after ray up is complete. This raises a bigger issue, what if the user wants to ssh to the machine in the middle of ray up.

This is a likely case, since initial users may lose patience when sky launch is taking too long and want to see what is wrong with their provisioned node.

Potential Fix (suggested by @Michaelvll): ray up with empty config to setup ~/.ssh/sky-key and ~/.ssh/config as soon as possible. Then, ray up with full yaml.

[Michaelvll]

This problem is largely mitigated by #400 and #407, since the user can ssh into the instance even the setup or file_mounts fails. Closing, please open another one if the problem still exists.