Add support for 'connection' concept

This is a large refactor and as small as I could feasibly make it
while keeping the tests working. I'll do the documentation and
touch ups in the next commit to make digesting easier.

Change-Id: Iac5083996a183d1d8a9b6cb8f70836f7c39ee910
diff --git a/tests/fixtures/layouts/bad_pipelines6.yaml b/tests/fixtures/layouts/bad_pipelines6.yaml
index aa91c77..bf2d538 100644
--- a/tests/fixtures/layouts/bad_pipelines6.yaml
+++ b/tests/fixtures/layouts/bad_pipelines6.yaml
@@ -2,7 +2,7 @@
   - name: check
     manager: IndependentPipelineManager
     trigger:
-      gerrit:
+      review_gerrit:
         - event: comment-added
           # approved is not a valid entry. Should be approval.
           approved: 1