Add missing input defaults

This change adds some missing defaults to the Input schemas to
improve it's usability, resulting in this expression:

  Input::{ name = "my-zuul" }

To fail with this message:

  Expression doesn't match annotation

  { executor : { - ssh_key : …
               , …
               }
  , launcher : { - config : …
               , …
               }
  , scheduler : { - config : …
                , …
                }
  , …
  }

Instead of:

  { - connections : …
  , - executor : …
  , - launcher : …
  , - scheduler : …
  , …
  }

Change-Id: I0d1c60878f95fbd559b8de4d2cbf5ddfa9da8667

conf/zuul/input.dhall

@@ -131,6 +131,10 @@ let Input =
           , external_config = Schemas.ExternalConfigs.default
           , merger = Schemas.Merger.default
           , web = Schemas.Web.default
+          , scheduler = Schemas.Scheduler.default
+          , executor = Schemas.Executor.default
+          , launcher = Schemas.Launcher.default
+          , connections = Schemas.Connections.default
           }
       }