Creating ami should be idempotent

If the ami already exists, return details of the duplicate
ami instead of failing with an error.

cloud/amazon/ec2_ami.py

@@ -20,7 +20,7 @@ module: ec2_ami
 version_added: "1.3"
 short_description: create or destroy an image in ec2
 description:
-     - Creates or deletes ec2 images. 
+     - Creates or deletes ec2 images.
 options:
   instance_id:
     description:
@@ -162,7 +162,16 @@ def create_image(module, ec2):
 
         image_id = ec2.create_image(**params)
     except boto.exception.BotoServerError, e:
-        module.fail_json(msg = "%s: %s" % (e.error_code, e.error_message))
+        if e.error_code == 'InvalidAMIName.Duplicate':
+            images = ec2.get_all_images()
+            for img in images:
+                if img.name == name:
+                    module.exit_json(msg="AMI name already present", image_id=img.id, state=img.state, changed=False)
+                    sys.exit(0)
+            else:
+                module.fail_json(msg="Error in retrieving duplicate AMI details")
+        else:
+            module.fail_json(msg="%s: %s" % (e.error_code, e.error_message))
 
     # Wait until the image is recognized. EC2 API has eventual consistency,
     # such that a successful CreateImage API call doesn't guarantee the success