title: boto3 wait_until_running doesn't work as desired
body: <p>I'm trying to write a script using boto3 to start an instance and wait until it is started. As per the documentation of wait_until_running, it should wait until the instance is fully started (I"m assuming checks should be OK) but unfortunately it only works for wait_until_stopped and incase of wait_until_running it just starts the instance and doesn't wait until it is completely started. Not sure if I'm doing something wrong here or this is a bug of boto3.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>import boto3
  
  
  ec2 = boto3.resource('ec2',region_name="ap-southeast-2")
  ec2_id = 'i-xxxxxxxx'
  instance = ec2.Instance(id=ec2_id)
  print("starting instance " + ec2_id)
  instance.start()
  instance.wait_until_running()
  print("instance started")
  </code></pre>