Remove hard-coded storage flags from pbs.py

pbs.py has the storage flag hard-coded, these should be abstracted out to either the internal.py (for defaults) or incoming from the config.yml (as an override) so that storage changes do not require a code change.

https://github.com/CABLE-LSM/benchcab/blob/7ee1b7b1df0eebd2a20cb278118dea66926ecf8b/src/benchcab/utils/pbs.py#L36