rapi: fix tag operations

This patch fixes the tag PUT/DELETE operations, and additionally changes
the _Tags_* functions to take only positional and not keyword arguments
(the defaults do not make any sense at all, and they are always called
with all arguments).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>