Allow Jenkins jobs to be only notified on failure

Summary:
In the Jenkins configuration file, any job in notifyOnlyOnFailure
will be ignored if the event hasn't a failure status.

This allows to handle at notifications level what the notifications
plugin doesn't handle yet.

Ref T953.

Test Plan:

• Fire notification for a successful Jenkins task, see it ignored
• Unit tests

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T953

Differential Revision: https://devcentral.nasqueron.org/D628