More detailed helpers for node generators

Currently there's one function to generate job nodes: [create_job_node](https://github.com/kernelci/kernelci-core/blob/537a0f15f92c69cec2f30af67dd0bbd0908907ab/kernelci/api/helper.py#L104). We now have different node types, with different requirements. It makes sense to split this functionality into more specific helpers, it'd also help keep a better coupling between the model definitions and the node handling logic.

Start in kernelci/api/helper.py at create_job_node, then read the model definitions and existing node handling logic to identify the different node types and their requirements. The work is done when node generation is split into specific helpers with the model definitions and handling logic kept consistently coupled.

Written by the indexing model from the issue text.