vnf_relations VNF: missing alternative-sw-image-desc in one VDU

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

charm-packages/vnf_relations_vnf/vnf_relations_vnfd.yaml

@@ -110,6 +110,8 @@ vnfd:
           type: PARAVIRT
     name: simple_requires
     sw-image-desc: ubuntu18.04
+    alternative-sw-image-desc:
+    - ubuntu18.04-azure
     virtual-compute-desc: simple_requires-compute
     virtual-storage-desc:
     - simple_requires-storage