Automatic merge from submit-queue (batch tested with PRs 36990, 37494, 38152, 37561, 38136)

Node E2E: Move ssh related functions into ssh.go.

This PR moves all ssh related functions and variables into a separate file `ssh.go`.

This is a minor cleanup preparing for my test framework refactoring work. Will send out the refactor PR later.

/cc @kubernetes/sig-node