Add support for zuul.d configuration split

This change implements the zuul_split spec to support configuration split in
a zuul.d directory.

Change-Id: I6bc7250b2045b73dfba109aa0b2f1ba5d66752b2
diff --git a/tests/fixtures/config/split-config/main.yaml b/tests/fixtures/config/split-config/main.yaml
new file mode 100644
index 0000000..5f57245
--- /dev/null
+++ b/tests/fixtures/config/split-config/main.yaml
@@ -0,0 +1,9 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - common-config
+        untrusted-projects:
+          - org/project
+          - org/project1