Allow manual-only job triggering

[TomasTomecek]

TODO:

• introduce a new bool configuration option manual/manual_trigger (default False)
  • add this to the schema
  • add this to the configuration class
• filter the events based on the new job configuration option:
  • here is the code that gets the handlers for the particular events (e.g.comment, push)
    • (we are matching the trigger of the DB object corresponding to the event and the trigger defined in the configuration and then working with the mapping to handlers defined via decorators of the handler classes, example)
• implement this for the dependant jobs as well (build -> test)

More details: https://docs.google.com/document/d/1UTaOctS-ueJ9KtDrh0C-OY_IpFY4vcsFtONJ4kOd88U/edit

Related-ish: #1754

[lachmanfrantisek]

Maybe a more generic comment_only/manual_trigger/... flag for all the jobs.

[evgeni]

We (@theforeman) would also be interested in this :)

[fernflower]

Leapp team definitely wants this, some of our regression tests (sst ones) take eternity to finish, so we currently run them on-demand only for specific prs.