Follow-up to 35aecf4c9aa. Somehow I missed the fact that there's an ILM
API named `retry`, which is a keyword in Ruby. I've removed it from the
keywords list.
If an API name (or components of a name) overlaps with a reserved word in
the programming language for an ES client, then it's possible that the code
that is generated from the API will not compile. This PR adds validation to
check for such overlaps.