LU execution: implement dry-run framework

This patch adds a new (global) opcode flag 'dry_run' which, when True,
causes early exit from the LU workflow, returning a special value from
the LU object (initialized in the parent LogicalUnit class, and which if
not overriden from child LUs will be None).

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