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/good_layout.yaml b/tests/fixtures/layouts/good_layout.yaml
index 3608d0c..0e21d57 100644
--- a/tests/fixtures/layouts/good_layout.yaml
+++ b/tests/fixtures/layouts/good_layout.yaml
@@ -8,7 +8,7 @@
       open: True
       current-patchset: True
     trigger:
-      gerrit:
+      review_gerrit:
         - event: patchset-created
         - event: comment-added
           require-approval:
@@ -17,16 +17,16 @@
           approval:
             - workflow: 1
     success:
-      gerrit:
+      review_gerrit:
         verified: 1
     failure:
-      gerrit:
+      review_gerrit:
         verified: -1
 
   - name: post
     manager: IndependentPipelineManager
     trigger:
-      gerrit:
+      review_gerrit:
         - event: ref-updated
           ref: ^(?!refs/).*$
           ignore-deletes: True
@@ -46,32 +46,32 @@
       approval:
         - code-review: [-1, -2]
     trigger:
-      gerrit:
+      review_gerrit:
         - event: comment-added
           approval:
             - approved: 1
     start:
-      gerrit:
+      review_gerrit:
         verified: 0
     success:
-      gerrit:
+      review_gerrit:
         verified: 2
         code-review: 1
         submit: true
     failure:
-      gerrit:
+      review_gerrit:
         verified: -2
         workinprogress: true
 
   - name: merge-check
     manager: IndependentPipelineManager
-    source: gerrit
+    source: review_gerrit
     ignore-dependencies: true
     trigger:
       zuul:
         - event: project-change-merged
     merge-failure:
-      gerrit:
+      review_gerrit:
         verified: -1
 
 jobs: