title: Cannot start an AWS ssm session on EC2 Amazon linux instance
body: <p>I want to start an AWS SSM session to an EC2 Amazon linux machine. In my AWS Systems Manager preferences I have set <code>RunAs</code> user as <code>ubuntu</code> so basically I am looking for a command which would let me start a session with a different <code>RunAs</code> user.</p>
  <p>Something like this:</p>
  <pre><code>aws ssm start-session --target i-id --RunAs ec2-user
  </code></pre>
  <p>Current error message which I get on executing:</p>
  <pre><code>aws ssm start-session --target i-id
  
  ----------ERROR-------
  Unable to start command: failed to start pty since RunAs user ubuntu does not exist
  </code></pre>