Introduce an RPC call for OS parameters validation

While we only support the 'parameters' check today, the RPC call is
generic enough that will be able to support other checks in the future.
The backend function will both validate the parameters list (so as to
make sure we don't pass in extra parameters that the OS validation
doesn't care about) and the parameter values, via the OS verify script.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>