Update sendCommand to use WaitGroup

[Will Moore]

If instanceIDs is large, there's a reasonable chance that some command executions will complete sooner than others, and it's possible that some might take so much time that the service-side record of the command might be garbage-collected before we retrieve the execution status.

	// Wait for the sent commands to complete
	wg := sync.WaitGroup{}
	for _, v := range instanceIDs {
		wg.Add(1)
		go func(instanceID string) {
			  ssmClient.WaitUntilCommandExecuted(&ssm.GetCommandInvocationInput{
				  CommandId:  aws.String(commandID),
				  InstanceId: aws.String(instanceID),
			  })
			  wg.Done()
		}(aws.StringValue(v))
	}
	wg.Wait()

Originally posted by @samuelkarp in #34 (comment)