Modify batch-create to use MultiAlloc

This is the CLI part of the OpInstanceMultiAlloc change. There is some
room for improvement especially with user handling, but it works for now
and the code got simplified significantly.

QA tests still needs to be written.

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