Update several_volumes_vnf image for aws to be Ubuntu20.04

Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>

several_volumes_vnf/several_volumes_vnfd.yaml

@@ -24,7 +24,7 @@ vnfd:
     image: ubuntu20.04
   - id: ubuntu20.04-aws
     name: ubuntu20.04-aws
-    image: ubuntu/images/hvm-ssd/ubuntu-artful-17.10-amd64-server-20180509
+    image: ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20210503
     vim-type: aws
   - id: ubuntu20.04-azure
     name: ubuntu20.04-azure