Thats True!! Thanks Luc. It seems there is no other way 
Hope we will have "Tags" with clonevm_task in upcoming releases.
actually i was finding out the fastest way to clone a VM from a single template with all the requirements we need like CPU;Memory, network portgroup changes based on the input csv we provide. In this one of my milestone was to add custom attributes where i am stuck at.
Question is still open if any one can help..