Move status_expiry to webapp section

This is a webapp attribute, it doesn't belong in scheduler.

Change-Id: If24dd50d7f747ed6ac830764bb9c92a0b2a3fe2d
diff --git a/doc/source/admin/components.rst b/doc/source/admin/components.rst
index 1c639d6..c77c0fc 100644
--- a/doc/source/admin/components.rst
+++ b/doc/source/admin/components.rst
@@ -106,8 +106,6 @@
   Port on which the webapp is listening (default: 8001).
   ``port=8008``
 
-.. TODO: move this to webapp (currently in 'zuul')
-
 **status_expiry**
   Zuul will cache the status.json file for this many seconds. This is an
   optional value and ``1`` is used by default.
diff --git a/zuul/cmd/scheduler.py b/zuul/cmd/scheduler.py
index b32deaf..e89480b 100755
--- a/zuul/cmd/scheduler.py
+++ b/zuul/cmd/scheduler.py
@@ -155,8 +155,7 @@
 
         zookeeper.connect(zookeeper_hosts)
 
-        cache_expiry = get_default(self.config, 'zuul', 'status_expiry', 1)
-
+        cache_expiry = get_default(self.config, 'webapp', 'status_expiry', 1)
         listen_address = get_default(self.config, 'webapp', 'listen_address',
                                      '0.0.0.0')
         port = get_default(self.config, 'webapp', 'port', 8001)