Fix adjustement of candidates in cluster modify

The code for adjusting the candidate pool size was done after the config
update, and this means we triggered the save of the config file without
fixing the candidate pool, which aborts with an error.

The patch just moves it above. The old comment was valid, but we anyway
save the config file in MaintainCandidatePool, so this should be safe.

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