[FIXED JENKINS-18813] Add/RemoveTrigger now works

Ensure the addTrigger and removeTrigger methods only use methods that are
supported by DescribableList. This means no indexed operations are
supported, so use only Iterator#remove and Collection#add

Add Integration Tests to ensure methods work as expected
Merge pull request #880 from evilmilo/master

[FIXED JENKINS-18813] Add/RemoveTrigger now works