Use existing env variable for default prio

There already was an environment variable defined for default priorities
for work_columns(). For consistency, use that same variable instead of
introducing our own.
Change default default priority to 5

A default priority of 5 makes it easier to insert jobs or queuss with
either a lower or a higher priority than the default.
Minor typo fix
Rename weight default variable for consistency
Add minumum, maximum, weight to jobs
Actually use JOBQUEUE_MODEL_MAPPINGS