Automatic merge from submit-queue

Node E2E: Set user with `--ssh-user` flag when running remote node e2e.

This PR unblocks https://github.com/kubernetes/test-infra/issues/1348.

In our test environment, we must login test instance as user `jenkins` because of the service account. Node e2e is always using the default user on the host, which works fine till now, because it is always run as `jenkins` in our test environment.

However, now we moved the test runner into a docker container, inside the container user is `root` by default, which will cause error:
```
Permission denied (publickey)
```

This PR added a flag `--ssh-user` to explicitly specify the user used to ssh into test instance. The dockerized test runner can set user to `jenkins` with this flag.

@krzyzacy  @ixdy