Move “rapi_users” file into separate directory

This reduces the number of notifications in “ganeti-rapi”. Until now it
was notified for every change in …/lib/ganeti and had to check whether
the users file was affected. A symlink is always created in cfgupgrade
to not break tools referring to the old name.

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