PR builds are triggered and failing for merged PR

[dahorak]

Merged PR builds seems to also trigger CentOS CI PR build. But the triggered job fails, because the original commit/branch is no more available for merged PR. For example:
https://ci.centos.org/view/tendrl-PR/job/tendrl-build-1-pr-monitoring-integration/45/console

I see following possible solutions here:

1. enhance/limit the GitHub trigger functionality to not to send trigger for merged PR,
2. enhance the trigger capture filter on the Jenkins side, to ignore webhooks from merged PRs,
3. ensure, that the triggered job for merged PR will not fail (automatically pass it, or build it against master branch).

The solutions are sorted by priority, but I'm not sure if the first or second solution is possible. I'll investigate it and take action.