Fixing issue with instanceInitiatedShutdownBehavior when ami root_device...

..._type is instance-store

The error I get is the following while using ami ami-94cd60fd ( instance-store backed) . Looks like if  shutdown_behavior set to stop , which is the default , it doesn't work with instance-store images. I later tried with ami-bf5021d6 which is ebs backed and things worked fine.

Response: <?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidParameterCombination</Code><Message>The attribute instanceInitiatedShutdownBehavior can only be used for elastic images.</Message></Error></Errors><RequestID>a3b0989d-fe80-4218-97b6-ba7f3a961016</RequestID></Response>