qa/suites/rados/thrash-erasure-code-big/thrashers: add `osd max backfills` setting...

qa/suites/rados/thrash-erasure-code-big/thrashers: add `osd max backfills` setting to mapgap and pggrow

All `rados/thrash-erasure-code-big` tests that die due to the “wait_for_recovery” timeout have one thing in common: They contain either `thrashers/pggrow` or `thrashers/mapgap`.

The difference between pggrow and mapgap vs. all other non-offending thrashers (default, careful, fastread, and morepggrow) is that they lack an override setting for `osd max backfills`. `osd max backfills` is the max number of backfill operations allowed to/from an OSD. The higher the number, the quicker the recovery. By default, this value is 1. On all of the non-offending thrashers (default, careful, fastread, and morepggrow), the default 1 value gets overridden in their .yaml files with a value > 1. This is not the case for pggrow and mapgap, however, as they lack an `osd max backfills` override setting.

The mclock op scheduler is known to override `osd max backfills` with a high value, but all of the thrash-erasure-code-big thrashers have their op queue set to “debug_random”, which chooses randomly between op queues (the debug_random op queue is set to override the default mclock_scheduler in qa/config/rados.yaml). So, coupled with the “debug_random” op queue, the low `osd max backfill` setting is causing some tests to time out in recovery.

WITHOUT `osd max backfills`, as they are now, “mapgap” and “pggrow” tests die due to timed-out recovery about 17/100 times, as seen here with a pggrow test: http://pulpito.front.sepia.ceph.com/lflores-2022-05-18_14:24:29-rados:thrash-erasure-code-big-master-distro-default-smithi/

WITH `osd max backfills` specified, as I have suggested in this PR, 99/100 tests passed, with one test failing for a different reason:
http://pulpito.front.sepia.ceph.com/lflores-2022-05-17_22:40:27-rados:thrash-erasure-code-big-master-distro-default-smithi/

I also scheduled 145 tests WITH `osd max backfills` that are a mix of pggrow and mapgap thrashers. 144/145 tests passed, with one test failing for a different reason. http://pulpito.front.sepia.ceph.com/lflores-2022-05-17_15:27:54-rados:thrash-erasure-code-big-master-distro-default-smithi/

Fixes: https://tracker.ceph.com/issues/51076
Signed-off-by: Laura Flores <lflores@redhat.com>