Normalise node status

Currently, depending on the Slurm connection method (cli vs api), the cluster nodes state comes in two formats:

1. "state": "idle" - Slurm CLI
2. "state": ["IDLE"] - Slurm API
3. "state": "free" - PBS

Furthermore the values of the state are not consistent across scheduler type e.g. idle for Slurm and free on PBS

Instead, a single format and values should be chosen and applied across schedulers.