splitgraph.yml generator: no lists for JSONSchema with `oneOf` with a single option

[mildbyte]

If a schema has a oneOf with only a single option, there's no need to generate a list with multiple potential options to go into this oneOf: https://github.com/splitgraph/sgr/blob/bc7ea998a490f33f4502112ac92540974d547e8b/splitgraph/cloud/project/generation.py#L86