Modifiy EbsOptimized param type in LauchConfig.

Since a couple of days ago, API expects a xsd1.1 boolean type for paramenter
EbsOptimized when LaunchConfiguration is created. This commit change old
True and False native python types for new lowercase string ones.

Currently boto fails with error:

 File "/home/ajdiaz/env/boto/lib/python2.7/site-packages/boto/connection.py", line 1069, in get_object
    raise self.ResponseError(response.status, response.reason, body)
BotoServerError: BotoServerError: 400 Bad Request
<ErrorResponse xmlns="http://autoscaling.amazonaws.com/doc/2011-01-01/">
  <Error>
    <Type>Sender</Type>
    <Code>MalformedInput</Code>
    <Message>boolean must follow xsd1.1 definition</Message>
  </Error>
  <RequestId>436a281a-c3ba-11e2-9cf1-e5c6f739aba9</RequestId>
</ErrorResponse>