Rename the JSON for GCEPersistentDisk in internal

I know the internal JSO tags are slated for death.  Until then, make them
correct.  Also fix some examples of v1b3 conversions that did not rename the
JSON field.

gce-pd/v1beta3/testpd.yaml

@@ -15,7 +15,7 @@ spec:
           mountPath: /testpd
   volumes:
     - name: testpd
-      persistentDisk:
+      gcePersistentDisk:
         # This GCE PD must already exist.
         pdName: %insert_pd_name_here%
         fsType: ext4

mysql-wordpress-pd/v1beta3/mysql.yaml

@@ -33,7 +33,7 @@ spec:
               mountPath: /var/lib/mysql
       volumes:
         - name: mysql-persistent-storage
-          persistentDisk:
+          gcePersistentDisk:
             # This GCE PD must already exist.
             pdName: mysql-disk
             fsType: ext4

mysql-wordpress-pd/v1beta3/wordpress.yaml

@@ -30,7 +30,7 @@ spec:
               mountPath: /var/www/html
       volumes:
         - name: wordpress-persistent-storage
-          persistentDisk:
+          gcePersistentDisk:
             # This GCE PD must already exist.
             pdName: wordpress-disk
             fsType: ext4