Allow retrying the initial SSH connection

[coledave]

Although the server is provisioned OpenSSH isn't always ready in time to receive the initial dial(), this causes the build to fail with a "Connection refused on host x.x.x.x:22" error.

I only know the basics of Go but this works for me, happy to amend as requested if it means getting it merged.

[bradrydzewski]

thanks for sending a pull request. Instead of putting all the retry logic inside the Run function inline, it would be better to wrap dial with a helper function like this:

-	client, err := dial(
+	client, err := retryDial(
		spec.ip,
		spec.Server.User,	
		e.privatekey,
	)

the dial with retry function could also accept a context so that if / when a user cancels a build the runner stops waiting for ssh to initialize.

// helper function configures and dials the ssh server and retries if
// there is an error connecting.
func retryDial(ctx context.Context, server, username, privatekey string) (*ssh.Client, error) {
	var client *ssh.Client
	var err error
	for i := 0;; i++ {
		client, err := dial(server, username, privatekey)
		if err == nil || i == 5 {
			break
		}
		select {
		case <- ctx.Done():
			return nil, ctx.Err()
		case <- time.After(time.Second * 10):
		}
	}
	return client, err
}

[CLAassistant]

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