schema: use pid, zone, stepwise schemas

These schemas were defined, but not referenced anywhere.  Use them, and
remove the duplicated properties from the legacy aggregate schema.

Change-Id: I75ba8099059127cfa5c2d6492908333ca5c3bf67
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/schemas/global.json b/schemas/global.json
index eb1af51..ee49869 100644
--- a/schemas/global.json
+++ b/schemas/global.json
@@ -9,6 +9,15 @@
             ],
             "oneOf": [
                 {
+                    "$ref": "Pid.json#/definitions/Pid"
+                },
+                {
+                    "$ref": "Pid.Zone.json#/definitions/PidZone"
+                },
+                {
+                    "$ref": "Stepwise.json#/definitions/Stepwise"
+                },
+                {
                     "$ref": "legacy.json#/definitions/Aggregate"
                 },
                 {