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_reject.yaml b/tests/fixtures/layouts/bad_reject.yaml
index b1e7f84..0549875 100644
--- a/tests/fixtures/layouts/bad_reject.yaml
+++ b/tests/fixtures/layouts/bad_reject.yaml
@@ -17,5 +17,5 @@
         - code-review: [-1, -2]
           username: core-person
     trigger:
-      gerrit:
+      review_gerrit:
         - event: patchset-created