Add the forward command to bmctool

[robertodauria]

This PR adds the bmctool forward to create an SSH tunnel to a BMC through a trusted host.

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This change is 

[stephen-soltesz]

This is really cool :D

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @evfirerob and @stephen-soltesz)

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cmd/forward.go, line 21 at r1 (raw file):

	tunnelHost string

	defaultPorts = []string{"4443:443", "5900"}

From a usability perspective, I will personally always use "443:443" to prevent odd behavior from the drac web UI. But, for default settings this is probably right.

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cmd/forward.go, line 130 at r1 (raw file):

		log.Infof("Forwarding %s -> %s -> %s", localEndpoint, serverEndpoint, remoteEndpoint)
		errs.Go(tunnel.Start)

This is awesome.

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tunnel/tunnel_test.go, line 17 at r1 (raw file):

func TestSSHTunnel_Start(t *testing.T) {
	handlerFunc := func(s sshserver.Session) {
		io.WriteString(s, "test")

Can this be an "echo" service? I was a little confused that "configure" resulted in "test".

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tunnel/tunnel_test.go, line 111 at r1 (raw file):

	_, err := sshIn.Write([]byte(cmd + "\r"))
	if err != nil {
		t.Errorf("Cannot write: %v", err)

Test execution doesn't stop on t.Errors - it would for t.Fatal -- can the test continue meaningfully after a failed Write?