Adding gcp image to vnfd

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

keep_persistent_volume_vnf/keep_persistent_volume_vnfd.yaml

@@ -30,6 +30,10 @@ vnfd:
     name: ubuntu20.04-azure
     image: Canonical:0001-com-ubuntu-server-focal:20_04-lts:latest
     vim-type: azure
+  - id: ubuntu20.04-gcp
+    name: ubuntu20.04-gcp
+    image: ubuntu-os-cloud:image-family:ubuntu-2004-lts
+    vim-type: gcp
   vdu:
   - id: keep-persistent-vol-VM
     name: keep-persistent-vol-VM