Fix deletion of individual attributes of bgp vrf resource (#286)

Author: tomvrugt

gen/definitions/router_bgp_vrf.yaml

@@ -111,9 +111,11 @@ attributes:
         example: false
       - yang_name: timers/keepalive-interval
         optional: true
+        delete_parent: true
         example: 5
       - yang_name: timers/holdtime
         optional: true
+        delete_parent: true
         example: 20
       - yang_name: update-source
         example: GigabitEthernet0/0/0/1