os: AWS: add allowed values to InstanceType

[philips]

Here is an example: https://gist.github.com/fivethreeo/b10ca1f36420629efa02#file-coreos-beta-pv-template-L49

As suggested by Øyvind here: https://groups.google.com/forum/#!searchin/coreos-dev/instancetype/coreos-dev/4ocb7H1C4t8/uv3KOUxXBwAJ

[robszumski]

For what its worth, we specifically removed the allowed values because it kept falling behind the new families of instances types that are always announced or expanded. I’m not sure which is better, but for now we’re allowing all input.

[pop]

Closing due to inactivity.