vnf stuck in PENDING_SCALE_OUT state while scaling up

1. Create vnf with following template.
tosca_definitions_version: tosca_simple_profile_for_nfv_1_0_0

description: sample-tosca-vnfd-scaling

metadata:
  template_name: sample-tosca-vnfd-scaling

topology_template:
  node_templates:
    VDU1:
      type: tosca.nodes.nfv.VDU.Tacker
      properties:
        image: cirros-0.3.4-x86_64-uec
        mgmt_driver: noop
        availability_zone: nova
        flavor: m1.tiny

    CP1:
      type: tosca.nodes.nfv.CP.Tacker
      properties:
        management: true
        anti_spoofing_protection: false
      requirements:
        - virtualLink:
            node: VL1
        - virtualBinding:
            node: VDU1

    VDU2:
      type: tosca.nodes.nfv.VDU.Tacker
      properties:
        image: cirros-0.3.4-x86_64-uec
        mgmt_driver: noop
        availability_zone: nova
        flavor: m1.tiny

    CP2:
      type: tosca.nodes.nfv.CP.Tacker
      properties:
        management: true
        anti_spoofing_protection: false
      requirements:
        - virtualLink:
            node: VL1
        - virtualBinding:
            node: VDU2

    VL1:
      type: tosca.nodes.nfv.VL
      properties:
        network_name: private
        vendor: Tacker

  policies:
    - SP1:
        type: tosca.policy.tacker.Scaling
        properties:
          increment: 2
          cooldown: 120
          min_instances: 1
          max_instances: 21
          default_instances: 2
          targets: [VDU1, VDU2]

2. Scale out vnf with following command,
tacker vnf-scale --vnf-id 49f7ff22-f56e-4fcf-a658-2088b1067cfd --scaling-policy-name SP1 --scaling-type out
3. On continues scaling, the vnf stucks at following stage.
vyatta@os-dev-06:/opt/stack/logs$ tacker vnf-list
+-------------------+-------------+-------------------+-------------------+-------------------+-------------------+-----------------------+--------------+
| id | name | description | mgmt_url | status | vim_id | placement_attr | error_reason |
+-------------------+-------------+-------------------+-------------------+-------------------+-------------------+-----------------------+--------------+
| 49f7ff22-f56e-4fc | Scaling VNF | sample-tosca- | {"VDU1": | PENDING_SCALE_OUT | 78c89d97-c85b- | {u'vim_name': | |
| f-a658-2088b1067c | | vnfd-scaling | ["15.0.0.4", | | 450e- | u'VIM0'} | |
| fd | | | "15.0.0.8", | | 8e17-e10c3713f88c | | |
| | | | "15.0.0.10", | | | | |
| | | | "15.0.0.14", | | | | |
| | | | "15.0.0.6", | | | | |
| | | | "15.0.0.17", | | | | |
| | | | "15.0.0.16", | | | | |
| | | | "15.0.0.25"], | | | | |
| | | | "VDU2": | | | | |
| | | | ["15.0.0.19", | | | | |
| | | | "15.0.0.21", | | | | |
| | | | "15.0.0.5", | | | | |
| | | | "15.0.0.3", | | | | |
| | | | "15.0.0.9", | | | | |
| | | | "15.0.0.7", | | | | |
| | | | "15.0.0.18", | | | | |
| | | | "15.0.0.23"]} | | | | |
+-------------------+-------------+-------------------+-------------------+-------------------+-------------------+-----------------------+--------------+
vyatta@os-dev-06:/opt/stack/logs$