Fixes #2028: LaunchConfiguration does not retrieve AssociatePublicIpAddress properly

Pull request #1799 added support to AssociatePublicIpAddress, but it added only support for sending that parameter. Retrieval was not fully supported yet.

This pull request is a ripoff of pull request #1832: https://github.com/boto/boto/pull/1832/files#diff-8c9af36969b22e4d4bb34924adc35399R105

I didn't add tests because I couldn't find an example and I am not familiar with boto's codebase.