Migrate from mitchellh/goamz to awslabs/aws-sdk-go in line with upstream Packer

[mefellows]

See hashicorp/packer/pull/2034 for detail, we need to get in line for compatibility.

[dylanmei]

Have to deal with the fallout of "config interpolation" hashicorp/packer#2145 in order to even get to this. Started a branch interpolate_configs as its far too much for a quick fixup.