Move the UsesRPC decorator from cli to rpc

This is needed because not just the cli scripts need this decorator, but
the master daemon too (and it already duplicated the code once).

In cli.py we just leave a stub, so that we don't have to modify all the
scripts to import rpc.py.

We then change the master daemon code to reuse this decorator, instead
of duplicating it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>