Return None if no suitable datastore is found

In the case of a datastore cluster, if no datastor is found then
return None. Do not fail on a key exception for accessing a field
that does not exist

Change-Id: I036f8022e2b9f454f7cd10846b408153a18b7b60

oslo_vmware/objects/datastore.py

@@ -61,6 +61,9 @@ def get_recommended_datastore(session, sp_spec):
         "RecommendDatastores",
         session.vim.service_content.storageResourceManager,
         storageSpec=sp_spec)
+    if not hasattr(spr, 'recommendations'):
+        LOG.error("Unable to find suitable datastore")
+        return
     return spr.recommendations[0].key