Make it possible to overwrite/manipulate job template priority on isos posts
The user story is:
For Maintenance we have a lot of incidents that are all for the same job group as they are incidents
e.g. for Leap 42.3/SLE 12 SP3. But the incidendents have different priorities - e.g. security issues
having nicknames are more important than random bug fixes.
But the openQA API only allows to create jobs with the job template group priority - and then manipulate
the job priorities one by one. While this is not a big problem, it would be good also for rescheduling
the job post if we had a variable to define either an offset to the job template priority or an absolute
value right away.