manage.py validate reports problems then says 'all jobs seem to be valid'

[Zygmunt Krynicki]

This was demonstrated during the plainbox provider info session

After removing the ID field and running validate, some jobs could not be validated (printing a few warnings/errors) but then manage.py printed 'all jobs seem to be valid'

[Zygmunt Krynicki (zyga)]

This only happens if we remove the 'id' field. This is because that field is validated separately at a lower level. Other fields are not validated when we load the provider (like the broken one here) and are only validated by `manage.py validate`.

[Zygmunt Krynicki (zyga)]

I think that to fix this bug we need to:

1) Remove the requirement to have 'id' to instantiate a job definition (allowing broken definitions to exist as objects)
2) Reject invalid jobs / providers in typical operations (probably complaining loudly as we do now)
3) Allow manage.py validate to work as intended

[Zygmunt Krynicki (zyga)]

I've added a merge proposal with the initial implementation. I'll polish it a little and we can finish this task.