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/zuul/model.py b/zuul/model.py
index 436a9c8..f901c55 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -1848,7 +1848,9 @@
         return set()
 
     def updatesConfig(self):
-        if 'zuul.yaml' in self.files or '.zuul.yaml' in self.files:
+        if 'zuul.yaml' in self.files or '.zuul.yaml' in self.files or \
+           [True for fn in self.files if fn.startswith("zuul.d/") or
+            fn.startswith(".zuul.d/")]:
             return True
         return False