Fix error during node group template update
Fix error during node group template update

this is because an optional key is referenced in calling validation
function which results in KeyError when key doesn't exist, and it
does not ever actually get used.

Change-Id: I12be69b5caf81289c2b8070fab8d2083709d6cf7
Closes-Bug: #1672516

openstack/sahara,openstack/sahara