Fix zuul.(yaml|d) loading order and add tests

This change fixes the configloader to load file in a more logical order:
zuul.yaml, zuul.d, .zuul.yaml then .zuul.d.

This change also adds negative test for config path conflicts.

Change-Id: Ifbbfb4389b7aa0a641b95b5947abc8a56e362ee3
diff --git a/tests/fixtures/config/conflict-config/main.yaml b/tests/fixtures/config/conflict-config/main.yaml
new file mode 100644
index 0000000..208e274
--- /dev/null
+++ b/tests/fixtures/config/conflict-config/main.yaml
@@ -0,0 +1,8 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - common-config
+        untrusted-projects:
+          - org/project