machine: provide SSH shell access to created instances

[pmrowla]

dvc machine ssh should drop users into an ssh session on the remote instance (similar to docker-machine ssh or vagrant ssh).

Since we default to relying on terraform to auto-generate SSH keys, there is no convenient way for users to ssh into the remote machine (unless they manually read tfstate to get the private key, write their own PEM file, and use the correct ssh ... flags). DVC should automate all of that for the user.