tasks/thrashosds: support overrides

e.g.,

overrides:
  thrashosds:
    thrash_primary_affinity: false
...
tasks:
- install:
- ceph:
- thrashosds:
- workunit:
...

Needed for #9865

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 7e41c93ed894823cdc53a0eed214edd86a00fbdc)