Add types and parameters for OpInstanceCreate

This is a "big" opcode, so sending it separately.

A few types needed changing, and a few parameters were renamed to make
it more clear which are cluster-level and which are instance-level
parameters.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>