Merge configurations with multiple layout files

Add a test for multiple tenants with a partially shared config.

We might use something like this in OpenStack to define common
pipelines but then have separate tenant config files for groups
of projects.

Change-Id: I29dc9327e3d72d5f6797eb2c366c36fe5be8ea8e
diff --git a/tests/fixtures/config/multi-tenant/tenant-two.yaml b/tests/fixtures/config/multi-tenant/tenant-two.yaml
new file mode 100644
index 0000000..57ad64d
--- /dev/null
+++ b/tests/fixtures/config/multi-tenant/tenant-two.yaml
@@ -0,0 +1,29 @@
+pipelines:
+  - name: tenant-two-gate
+    manager: DependentPipelineManager
+    success-message: Build succeeded (tenant-two-gate).
+    source:
+      gerrit
+    trigger:
+      gerrit:
+        - event: comment-added
+          approval:
+            - approved: 1
+    success:
+      gerrit:
+        verified: 2
+        submit: true
+    failure:
+      gerrit:
+        verified: -2
+    start:
+      gerrit:
+        verified: 0
+    precedence: high
+
+projects:
+  - name: org/project2
+    check:
+      - project2-test1
+    tenant-two-gate:
+      - project2-test1