Add cmdliner CLI using ocaml-rpc

[gaborigloi]

A CLI would help us double-check that the RPC function works and we properly call the Python scripts.

[edwintorok]

How about sm-cli?

[gaborigloi]

That would work too, but sm-cli tests more layers than that, it goes through the SMAPIv2->v3 translation layer.
If we added a CLI to xapi-storage-script, that would only test the interface between it and the SMAPIv3 Python scripts.