ceph-*trigger jobs need reworking

[zmc]

Fundamentally, the GitHub Pull Request Builder plugin is problematic because it may build an unknown number of branches in response to a single push event. It's also unmaintained.

ceph-dev-pipeline-trigger uses the generic git trigger plugin, which is more predictable, and maintained.

Edit: this issue specifically refers to ceph.git and ceph-ci.git builds; not builds of other related projects.

[zmc]

I made a proof-of-concept using the generic-webhook-trigger plugin. It was pretty quick to implement and its behavior is easier to understand than ghprb's. Currently it no-ops instead of actually triggering. I'd love feedback.

#2372

[zmc]

#2372 is merged - we're planning on doing a trial cut-over on Monday.