Add restrictions to the pull_request trigger

[dibbles]

Currently pull_requests trigger pipelines under a number of different scenarios, for example creation/label/unlabel/close ..... restrict the trigger to run only on pull_request creation and when subsequent pushes happen that update the code backing the pull request.

[vtereso]

/assign

[vtereso]

Within tektoncd/triggers#49, we will be using CEL to provide filtering within EventListener. Once this is in, I will update our generated EventListener to only trigger on [opened, synchronize] actions.

[dibbles]

That would mean this piece would have a reliance on triggers 0.2 presumably .... therefore we need a decision about our 0.3 release. Presuming it is triggers 0.1 then we would need to do some other implementation in our interceptor ....... then for 0.2 change code to exploit new triggers functionality.

Or we do nothing for our 0.3 release in this area ..... or we do 0.3 on 0.2 triggers.
@mnuttall decision please.

[vtereso]

While we await the Triggers release, I will add this filtering as a basic match within the interceptor.

[dibbles]

check in with @akihikokuroda - looks like there was another existing issue for this (I forgot I'd opened this) which he might have picked up #250