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_misplaced_ref.yaml b/tests/fixtures/layouts/bad_misplaced_ref.yaml
index f009c39..d8bb6bc 100644
--- a/tests/fixtures/layouts/bad_misplaced_ref.yaml
+++ b/tests/fixtures/layouts/bad_misplaced_ref.yaml
@@ -2,7 +2,7 @@
   - name: 'check'
     manager: IndependentPipelineManager
     trigger:
-      gerrit:
+      review_gerrit:
         - event: patchset-created
           ref: /some/ref/path