Only ssh into running instances

Change-Id: I7e6d13bded06b4ed443f798bca380e51efdf9374
Add better error messages for missing creds

Change-Id: I3372dc3a19f96458b42677d559f1539502f4d23c
Use exec instead of leaving a waiting ruby process

Change-Id: Ia9c190b96803a6950d895e99a2546879329e1f35