Don't assume we always SSH as the current user

This works on gcloud (where the user is dynamically created by the tool),
but doesn't hold on other clouds (e.g. AWS).

The function in pkg/util now takes a user arg, and it is called only
from the e2e tests, which now check for env-var KUBE_SSH_USER, and then
fall back to the existing behaviour of env-var USER.

I am using this from Jenkins by directly setting the env-var:

export KUBE_SSH_USER=jenkins
...
hack/jenkins/e2e.sh

pkg/util/ssh.go

@@ -142,21 +142,42 @@ func (s *SSHTunnel) Close() error {
 	return nil
 }
 
+// Interface to allow mocking of ssh.Dial, for testing SSH
+type sshDialer interface {
+	Dial(network, addr string, config *ssh.ClientConfig) (*ssh.Client, error)
+}
+
+// Real implementation of sshDialer
+type realSSHDialer struct{}
+
+func (d *realSSHDialer) Dial(network, addr string, config *ssh.ClientConfig) (*ssh.Client, error) {
+	return ssh.Dial(network, addr, config)
+}
+
 // RunSSHCommand returns the stdout, stderr, and exit code from running cmd on
-// host along with any SSH-level error.
-func RunSSHCommand(cmd, host string, signer ssh.Signer) (string, string, int, error) {
+// host as specific user, along with any SSH-level error.
+// If user=="", it will default (like SSH) to os.Getenv("USER")
+func RunSSHCommand(cmd, user, host string, signer ssh.Signer) (string, string, int, error) {
+	return runSSHCommand(&realSSHDialer{}, cmd, user, host, signer)
+}
+
+// Internal implementation of runSSHCommand, for testing
+func runSSHCommand(dialer sshDialer, cmd, user, host string, signer ssh.Signer) (string, string, int, error) {
+	if user == "" {
+		user = os.Getenv("USER")
+	}
 	// Setup the config, dial the server, and open a session.
 	config := &ssh.ClientConfig{
-		User: os.Getenv("USER"),
+		User: user,
 		Auth: []ssh.AuthMethod{ssh.PublicKeys(signer)},
 	}
-	client, err := ssh.Dial("tcp", host, config)
+	client, err := dialer.Dial("tcp", host, config)
 	if err != nil {
-		return "", "", 0, fmt.Errorf("error getting SSH client to host %s: '%v'", host, err)
+		return "", "", 0, fmt.Errorf("error getting SSH client to %s@%s: '%v'", user, host, err)
 	}
 	session, err := client.NewSession()
 	if err != nil {
-		return "", "", 0, fmt.Errorf("error creating session to host %s: '%v'", host, err)
+		return "", "", 0, fmt.Errorf("error creating session to %s@%s: '%v'", user, host, err)
 	}
 	defer session.Close()
 
@@ -176,7 +197,7 @@ func RunSSHCommand(cmd, host string, signer ssh.Signer) (string, string, int, er
 		} else {
 			// Some other kind of error happened (e.g. an IOError); consider the
 			// SSH unsuccessful.
-			err = fmt.Errorf("failed running `%s` on %s: '%v'", cmd, host, err)
+			err = fmt.Errorf("failed running `%s` on %s@%s: '%v'", cmd, user, host, err)
 		}
 	}
 	return bout.String(), berr.String(), code, err