Adding VNFD for Basic_28 keeping persistent volume feature

Signed-off-by: Gulsum Atici gulsum.atici@canonical.com

keep_persistent_volume_ns/keep_persistent_volume_nsd.yaml

+nsd:
+  nsd:
+  - description: NS with 1 VNFs connected to mgmtnet VL
+    df:
+    - id: default-df
+      vnf-profile:
+      - id: vnf-persistent-volumes
+        virtual-link-connectivity:
+        - constituent-cpd-id:
+          - constituent-base-element-id: vnf-persistent-volumes
+            constituent-cpd-id: vnf-mgmt-ext
+          virtual-link-profile-id: mgmtnet
+        vnfd-id: keep_persistent-volumes-vnf
+    id: persistent_volumes-ns
+    name: persistent_volumes-ns
+    version: 1.0
+    virtual-link-desc:
+    - id: mgmtnet
+      mgmt-network: true
+    vnfd-id:
+    - keep_persistent-volumes-vnf