PG::choose_acting: in mixed cluster case, acting may include backfill

Fixes: 9696
Backport: firefly, giant
Introduced: 92cfd370395385ca5537b5bc72220934c9f09026
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 9b18d99817c8b54e30dff45047dfe1b29871d659)