title: How to get arn of my ec2 instance using boto3
body: <p>I want the arn of all my ec2 instances. The describe_instances() does not give out the instances.
  Is there any other method or some way that may list out all the arns as well.
  I need them to be stored in a database,also I'm working on sample instances for now,this eventually needs to work for multiple accounts as well. So, a solution that will work throughout will be helpful.</p>