If user specifies a --associate-public-ip-address option and provides a --subnet-id option, we have to move the subnet ID value into the NetworkInterfaces structure we have created or we get a client error from EC2.  Fixes 501.
Incorporate a few review comments.
Give a better name to a test.
Merge branch 'fix-501' of git://github.com/garnaat/aws-cli into garnaat-fix-501